GIFT OF 
 'ofessor F.T. Biolett: 
 
 BtO'LOGY 
 
 'b., --: 
 
ELEMENTS 
 
 OF THE 
 
 COMPARATIVE ANATOMY 
 
 VEETEBEATES 
 
ELEMENTS 
 
 OF THE 
 
 COMPAEATIVE ANATOMY 
 
 OF 
 
 VEBTEBEATES 
 
 ADAPTED FROM THE GERMAN OF 
 
 DE. EOBEET WIEDEESHEIM 
 
 PROFESSOR OF ANATOMY, AND DIRECTOR OF THE INSTITUTE OF HUMAN AND COMPARATIVE ANATOMY 
 IX THE UNIVERSITY OF FREIBURG-IN-BADEN 
 
 BY 
 
 W. N. PARKER, PH.D. 
 
 PROFESSOR OF BIOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE 
 IN THE UNIVERSITY OF WALES 
 
 SECOND EDITION 
 
 (FOUNDED ON THE THIRD GERMAN EDITION) 
 
 WITH THREE HUNDRED AND THIRTY-THREE WOODCUTS 
 AND A BIBLIOGRAPHY. 
 
 ILonfcon 
 
 MACMILLAN AND CO., LIMITED 
 
 NEW YORK: THE MACMILLAN COMPANY 
 
 1897 
 
 All rights reserved 
 
RICHARD CLAY AND SONS, LIMITED 
 
 LONDON AND BUNGAY. 
 
 GIFT OF 
 
 
PBEFACE TO THE FIEST EDITION 
 
 PKOFESSOR WIEDERSHEIM'S Grundriss der vergleichenden 
 Anatomic der Wirbelthiere, published at Jena in 1884, was written 
 to supply a need which had been felt for some time past for a short 
 text-book on Vertebrate Anatomy embodying some of the more 
 recent views on the subject. The present book is a modified 
 translation of the Grundriss, and it is hoped that it will serve to 
 render Professor Wiedersheim's work more widely kuown amongst 
 English students. 
 
 The plan of the original has been retained throughout, though 
 numerous additions and modifications have been made to the work ; 
 for many of these I have to thank Professor Wiedersheim, for 
 others I am myself responsible. I must also express my 
 indebtedness to Professor Wiedersheim for revising the whole 
 translation with me last summer, and for much help while the 
 work was in progress. 
 
 Within the limits of a short text-book like the present, much 
 of the matter is of necessity greatly condensed : more detailed 
 accounts of the various parts and organs will be found in the new 
 edition of Professor Wiedersheim's Lehrbuch der vcrgl. Anatomie 
 der Wirbeltliicre, which is to appear shortly, and on the first edition 
 of which the G-rundriss was founded. 
 
 M30848 
 
vi PREFACE 
 
 The brevity of the descriptions is, however, to some extent 
 made up for by the number of woodcuts. Most of these are taken 
 from the German edition, but several new figures have been 
 added. 
 
 The arrangement of the book according to organs, and not 
 according to groups of animals, is likely to render it more difficult 
 for a beginner, and a general knowledge of Zoology will be of 
 great assistance. The pages on which the different groups are 
 described are, however, collected together in the index, so that the 
 sections relating to any one group can be easily referred to. The 
 present arrangement seems to be the only possible one if the book 
 is to be founded on a scientific basis, for it is most important that 
 the student should grasp the fact that there has been an evolution 
 of organs, as well as of animals. 
 
 The more theoretical and detailed matter is printed in small 
 type, and in the form of notes : the student should in most cases 
 pass this over when reading the book for the first time. A black 
 and a spaced type have been used to render prominent important 
 words or sentences. 
 
 A bibliography is appended at the end of each chapter. This 
 in no case presumes to be anything like a complete list of the 
 literature of the subject : our object has been more particularly 
 to mention the recent and the more important works, though many 
 of these have doubtless been omitted. References to other re- 
 searches can be found by consulting the works mentioned. 
 
 At Professor Wiedersheim's suggestion, I have not inserted a 
 translation of the preface to the original, as it seemed unnecessary 
 so to do. I may, however, mention that the book was written for 
 students of Medicine, as well as for those of Comparative Anatomy : 
 the intimate connection of the two subjects renders it most 
 
PREFACE vii 
 
 important that medical students should have a general scientific 
 basis for their special anatomical knowledge. 
 
 My sincerest thanks are due to my friends Professors F. Jeffrey 
 Bell and G. B. Howes, who have kindly read through the proof- 
 sheets. To them I am indebted for numerous valuable suggestions, 
 as well as for correcting many faults of style and expression which 
 had escaped my notice. I must also express my thanks to my 
 father, Professor W. K. Parker, and to Dr. Gadow, for many special 
 details in connection with the skeleton, as well as to Mr. E. 
 Radford for help in making the index. 
 
 W. N. PARKER. 
 UNIVERSITY COLLEGE, CARDIFF, 
 May, 1886. 
 
PEEFACE TO THE SECOND EDITION 
 
 SINCE the publication of the first edition of the Grundriss, on 
 which the first English edition was founded, two further German 
 editions have appeared, one in 1888 and another in 1893, the 
 latter containing 695 pages as compared with 272 pages in the 
 first edition. The book has, in fact, grown beyond the limits of 
 a "Grundriss," and has replaced the original Lehrbuch, no new edition 
 of which has appeared since 1886. 
 
 As it seemed desirable that the second English edition 
 should be brought up to date without greatly exceeding the 
 limits of the first, it has been necessary to use a free hand 
 in abridging and recasting the text. I have therefore, with 
 the author's permission, attempted to prepare a short text- 
 book which, while retaining the original descriptions and 
 arrangement as far as possible, should deal with the more 
 essential and well-ascertained facts of Comparative Anatomy, 
 presenting an approximate equality of treatment as regards its 
 different sections without entering too fully upon doubtful 
 theories or special details in Embryology and Physiology. 
 
 The book has thus been almost entirely rewritten, with the 
 approval of Professor Wiedersheim, who, besides revising the 
 work, has furnished me with much new material. A number 
 
PREFACE ix 
 
 of the old figures have been replaced and several additional 
 ones inserted. 
 
 The bibliography appended to the book, which has been 
 considerably added to by Professor Wiedersheim since the third 
 German edition was published, is rather extensive for a work 
 of the kind, but I have not ventured to make selections from it 
 and have merely modified the arrangement in some respects and 
 made a few additions which seemed to me important for English 
 readers. It will, I trust, be found useful by more advanced students. 
 
 I must acknowledge my obligations to my brother, Professor 
 T. Jeffery Parker, F.R.S., for numerous suggestions, and also 
 to Professor G. B. Howes, F.R.S., Mr. Frank J. Cole, and Mr. 
 Martin F. Woodward for valuable information on several special 
 points. 
 
 W. N. PARKER. 
 UNIVERSITY COLLEGE, CARDIFF, 
 
 April, 1897. 
 
CONTENTS 
 
 PAGE 
 
 Preface to the First Edition v 
 
 Preface to the Second Edition >,..'.: viii 
 
 INTRODUCTION 1 
 
 I. On the Meaning and Scope of Comparative Anatomy 1 
 
 II. Development and Structural Plan of the Vertebrate Body 2 
 
 III. Classified List of the Principal Vertebrate Groups , 13 
 
 IV. Table showing the Gradual Development of the Vertebrata in Time . 15 
 
 SPECIAL PART. 
 
 A. INTEGUMENT 16 
 
 of Amphioxus, Fishes, and Dipnoans 16 
 
 of Amphibians 18 
 
 of Reptiles 20 
 
 of Birds 20 
 
 of Mammals , . . 23 
 
 Mammary Glands 27 
 
 B. SKELETON 30 
 
 1. EXOSKELETON 30 
 
 2. ENDOSKELETON 34 
 
 I. VERTEBRAL COLUMN 34 
 
 of Fishes and Dipnoans 36 
 
 of Amphibians 42 
 
 of Reptiles 45 
 
 of Birds 47 
 
 of Mammals 49 
 
 II. RIBS 52 
 
 of Fishes and Dipnoans 54 
 
 of Amphibians 55 
 
 of Reptiles 56 
 
 of Birds . ...... .' . ' . . . ... . . . ; .\i , 56 
 
 of Mammals 57 
 
xii CONTENTS 
 
 VAOR 
 
 III. STERNUM 58 
 
 IV. EPISTERNUM 62 
 
 Y. SKULL 64 
 
 Introduction 64 
 
 a. Brain-case (cranium) 67 
 
 6. Visceral Skeleton .. 69 
 
 c. Bones of the Skull 70 
 
 Anatomy of the Skull (special part) 72 
 
 A. The Skull of Fishes 72 
 
 B. of Dipnoans 81 
 
 C. ,, of Amphibians 82 
 
 D. ,, of Reptiles .' 88 
 
 E. ,, of Birds 93 
 
 F. ,, of Mammals 96 
 
 VI. LIMBS 102 
 
 a. Unpaired Fins 102 
 
 I. Paired Fins or Limbs 103 
 
 Pectoral Arch 106 
 
 of Fishes and Dipnoans 106 
 
 of Amphibians 107 
 
 of Reptiles 108 
 
 of Birds 109 
 
 of Mammals 109 
 
 Pelvic Arch 109 
 
 of Fishes 109 
 
 of Dipnoans Ill 
 
 of Amphibians Ill 
 
 of Reptiles 114 
 
 of Birds 119 
 
 of Mammals '. 120 
 
 Free Limbs 122 
 
 of Fishes and Dipnoans 122 
 
 Phylogeny of the Ichthyopterygium 124 
 
 General Considerations on the Limbs of the higher Vertebrata 125 
 
 Free Limbs of Amphibians 127 
 
 of Reptiles 127 
 
 of Birds 129 
 
 of Mammals 130 
 
 C, MUSCULAR SYSTEM 135 
 
 INTEGUMENTARY MUSCULATURE 136 
 
 MUSCLES OF THE TRUNK 137 
 
 of Amphioxus, Fishes, and Dipnoans 137 
 
 of Amphibians 137 
 
 of Reptiles 138 
 
CONTENTS xiii 
 
 PAGE 
 
 MUSCLES OF THE TRUNK (continued) 
 
 of Birds 140 
 
 of Mammals 140 
 
 MUSCLES OF THE DIAPHRAGM 141 
 
 MUSCLES OF THE APPENDAGES 142 
 
 EYE-MUSCLES 142 
 
 VISCERAL MUSCLES 142 
 
 of Fishes 142 
 
 of Amphibia 143 
 
 of Amniota 144 
 
 D, ELECTEIC ORGANS 146 
 
 E, NERVOUS SYSTEM AND SENSORY ORGANS 149. 
 
 I. CENTRAL NERVOUS SYSTEM 149 
 
 MEMBRANES OF THE BRAIN AND SPINAL CORD 151 
 
 . 1. SPINAL CORD 152 
 
 2. BRAIN (general description and development) 153 
 
 of Cyclostomi 157 
 
 of Elasmobranchii and Holocephali 159 
 
 of Ganoidei 162 
 
 of Teleostei 162 
 
 of Dipnoi 165 
 
 of Amphibia 166 
 
 of Reptiles 167 
 
 of Birds , . . . . 172 
 
 of Mammals 172 
 
 II. PERIPHERAL NERVOUS SYSTEM 177 
 
 1. SPINAL NERVES 179 
 
 2. CEREBRAL NERVES ...... . . 180 
 
 Sympathetic 188 
 
 III. SENSORY ORGANS (general description and development) ... 189 
 
 SENSE-ORGANS OF THE INTEGUMENT 190 
 
 a. Nerve-Eminences 190 
 
 6. End-Buds 193 
 
 c. Tactile Cells and Corpuscles 195 
 
 d. Club-shaped Corpuscles 195 
 
 OLFACTORY ORGAN (general description and development) .... 196 
 
 of Cyclostomes 197 
 
 of Fishes 198 
 
 of Dipnoans 199 
 
 of Amphibians 200 
 
 of Reptiles 201 
 
 of Birds 202 
 
 of Mammals 203 
 
 Jacobson's Organ . 205 
 
xiv CONTENTS 
 
 PAGE 
 
 EYE (general description and development) 207 
 
 of Cyclostomes 211 
 
 of Fishes and Dipnoans 211 
 
 of Amphibians 212 
 
 of Reptiles and Birds 213 
 
 of Mammals 214 
 
 Retina 214 
 
 Accessory Organs in Connection with the Eye 216 
 
 a. Eye-Muscles 216 
 
 6. Eyelids 217 
 
 c. Glands '...,... 217 
 
 AUDITORY ORGAN (general description and development) 220 
 
 of Cyclostomes 224 
 
 of Fishes and Dipnoans 224 
 
 of Amphibians 226 
 
 of Reptiles and Birds 227 
 
 of Mammals 229 
 
 F. ORGANS OF NUTRITION 235 
 
 ALIMENTARY CANAL AND ITS APPENDAGES (general descrip- 
 tion) 235 
 
 i. MOUTH 239 
 
 Teeth (general description) 239 
 
 of Fishes, Dipnoans, and Amphibians 241 
 
 of Reptiles and Birds ' 243 
 
 of Mammals 245 
 
 Glands of the Mouth 250 
 
 of Amphibians 251 
 
 of Reptiles 251 
 
 of Birds 252 
 
 of Mammals 252 
 
 Tongue 252 
 
 THYROID 255 
 
 THYMUS 256 
 
 II. (ESOPHAGUS, STOMACH, AND INTESTINE 257 
 
 of Ichthyopsida . . 257 
 
 of Reptiles 262 
 
 of Birds 262 
 
 of Mammals 263 
 
 Histology of the Mucous Membrane of the Alimentary Canal . 267 
 
 LIVER ' 269 
 
 PANCREAS 272 
 
 G. ORGANS OF RESPIRATION 273 
 
 I. GILLS 273 
 
 of Amphioxus 275 
 
 of Cyclostomes 275 
 
CONTENTS xv 
 
 PAGE 
 
 GILLS (continued) 
 
 of Fishes . 276 
 
 of Dipnoans 278 
 
 of Amphibians 279 
 
 II. AIR-BLADDER AND LUNGS . . . 280 
 
 1. AIR-BLADDER 280 
 
 2. LUNGS .281 
 
 Air Tubes and Larj r nx 283 
 
 of Dipnoans 283 
 
 of Amphibians 283 
 
 of Reptiles >, . 284 
 
 of Birds 285 
 
 of Mammals , 286 
 
 Lungs proper 288 
 
 of Dipnoans ; 288 
 
 of Amphibians 288 
 
 of Reptiles ..;.... 290 
 
 of Birds 291 
 
 of Mammals 296 
 
 ABDOMINAL PORES 298 
 
 H. ORGANS OF CIRCULATION 299 
 
 General Description and Development 299 
 
 Heart, together with Origin of Main Vessels 305 
 
 of Fishes 305 
 
 of Dipnoans 307 
 
 of Amphibians 309 
 
 of Reptiles 313 
 
 of Birds and Mammals 315 
 
 Arterial System 319 
 
 Venous System 322 
 
 of Fishes . ... 322 
 
 of Dipnoans 326 
 
 of Amphibians .......... 328 
 
 of Amniota 328 
 
 Retia Mirabilia 333 
 
 Lymphatic System 333 
 
 MODIFICATIONS FOR THE INTER-UTERINE NUTRITION OF 
 
 THE EMBRYO : FCETAL MEMBRANES 336 
 
 1. Anamnia 336 
 
 2. Amniota 337 
 
 I. URINOGENITAL ORGANS 341 
 
 General Description and Development 341 
 
 Male and Female Generative Ducts 347 
 
 Gonads . 347 
 
xvi CONTENTS 
 
 URINARY ORGANS P 349 
 
 of Amphioxus 349 
 
 of Cyclostomes, Fishes and Dipnoans 350 
 
 of Amphibians 352 
 
 of Reptiles and Birds 356 
 
 of Mammals 358 
 
 GENERATIVE ORGANS 359 
 
 of Amphioxue 359 
 
 of Cyclostomes 359 
 
 of Fishes and Dipnoans 360 
 
 of Amphibians 365 
 
 of Reptiles and Birds 368 
 
 of Mammals 370 
 
 COPULATORY ORGANS 377 
 
 SUPRARENAL BODIES 385 
 
 APPENDIX (Bibliography) 389 
 
 INDEX . . 481 
 
COMPARATIVE ANATOMY 
 
 INTRODUCTION. , /; \ _ 
 
 I. ON THE MEANING AND SCOPE OF COMPARATIVE 'ANATOMY. 
 
 A KNOWLEDGE of the natural relationships and ancestral history 
 of animals can only be gained by a comparative study of their 
 parts (Comparative Anatomy) and of their mode of develop- 
 ment (Embryology or Ontogeny). In addition to existing 
 animals, fossil forms must also be taken into consideration ( Pa- 
 laeontology), and by combining the results obtained under these 
 three heads, it is possible to make an attempt to trace out the 
 development of the various races or groups in time (Phylogeny). 
 As the different phases of development of the race may be repeated 
 to a greater or less extent in those of the individual, the depart- 
 ments of Ontogeny and Phylogeny help to complete one another. 
 
 It must, however, be borne in mind that in many cases the 
 phases of development are not repeated accurately in the individual 
 that is, are not palingenetic, but that " falsifications " of the re- 
 cord, acquired by adaptation, very commonly occur along with 
 them, resulting in ccenogenetic modifications in which the original 
 relations are either no longer to be recognised at all, or are more 
 or less obscured. In this connection, two important factors must 
 be taken into consideration, viz., heredity and the capability of 
 variation. The former is conservative, and tends to the retention 
 of ancestral characters, while the latter, under the influence of 
 change in external conditions, results in modifications of structure 
 which are not fixed and unalterable, but are in a state of constant 
 change. The resulting " adaptations" so far as they are useful to 
 the organism concerned, are transmitted to future generations, 
 and thus in the course of time gradually lead to still further 
 modifications. Thus heredity and adaptation are parallel factors, 
 and a conception of the full meaning of this fact helps us not only 
 to gain an insight into the blood-relationships of animals in gene- 
 ral, but also to understand the meaning of numerous degenerated 
 
 B 
 
2 COMPARATIVE ANATOMY 
 
 and rudimentary or vestigial organs and parts in the adult organism 
 which would otherwise remain totally inexplicable. 
 
 Histology is a subdivision of anatomy which concerns the 
 structural elements the building-stones of the organism, and the 
 combination of these to form tissues. Various combinations of 
 the tissues give rise to organs, and the organs, again, combine to 
 form systems of organs. 
 
 The structural elements consist primarily of cells and second- 
 arily of cells and fibres, and the different tissues may be divided 
 into four principal groups : 
 
 1. Epithelium, and its derivative, glandular tissue. 
 
 2. Supporting-tissue (connective-tissue, cartilage, bone). 
 , . ,3. Muscular tissue. 
 
 . Nervoti> tissue. 
 
 ivitli the functions they perform, epithelium and support- 
 mg'ti'ssu& Inay be' described as passive, and muscular and nervous tissue as 
 active. 
 
 By an organ we understand an apparatus constructed to 
 perform a definite function : as, for instance, the liver, which 
 secretes bile ; the gills and lungs, in which an exchange of 
 gases is effected with the surrounding medium ; and the heart, 
 which pumps blood through the body. 
 
 The organ-systems, which will be treated of in order in this 
 book, are as follows : 1. The outer covering of the body, or inte- 
 gument ; 2. The skeleton ; 3. The muscles, together with electric 
 organs ; 4. The nervous system and sense-organs ; 5. The organs 
 of nutrition, respiration, circulation, excretion, and reproduction. 
 
 The closely-allied branches of science denned above are united 
 together as Morphology, as opposed to Physiology which con- 
 cerns the functions of organs, apart from their morphological rela- 
 tions. The results obtained from these two fields of study help to 
 complete one another, and thus to throw light on the organisation 
 of animals in general that is, on Zoology in its widest sense. 
 
 II. DEVELOPMENT AND STRUCTURAL PLAN OF THE 
 VERTEBRATE BODY. 
 
 The structural elements described in the preceding section as 
 the building-stones of the organism, i.e.. the cells, all arise from a 
 single primitive cell, the egg- cell or ovum. This forms the 
 starting-point for the entire anirnal-body, and a general account 
 of its structure and subsequent development must therefore be 
 given here. 
 
 The ovum consists cf a rounded vesicle (Fig. ]), in the interior 
 of which the following parts can be distinguished : the vitellus y 
 
INTRODUCTION 3 
 
 the germinal vesicle, and one or more germinal spots. The outer 
 covering of the ovum is spoken of as the vitelline membrane: 
 
 Since the ovum in its primitive form as above described repre- 
 sents a single cell, we may speak of the vitellus x as the protoplasm 
 of the egg-cell, the germinal vesicle as its nucleus, and the germinal 
 spot as its nucleolus. The cell-nucleus is enclosed by a delicate 
 nuclear membrane, and is made up of two constituents the 
 spongioplasm or chromatin, and the hyaloplasm or achromatin. 
 One or two small particles, the centrosomes, are also present in 
 the cell-body, and take an important part in the process of cell- 
 division. An outer limiting membrane, corresponding to the 
 vitelline membrane, is not an integral 
 part of the cell, but may be differen- 
 tiated as a hardening of the peripheral 
 protoplasm. 
 
 In sexual reproduction, such as 
 occurs in all Vertebrates, the fusion of 
 the sperm-cell, containing the genera- 
 tive substance of the male, with the 
 ovum, is an absolute necessity for the 
 development of the latter. FIG. 1. DIAGRAM OF THK 
 
 But before this can occur, certain UN-IMPREGNATED OVUM. 
 
 changes take place in the ovum, which z>, vitellus ; KB, germinal 
 are known as maturation. This con- vesicle ; KF, germinal spot, 
 sists of a twice-repeated process of cell- 
 division (karyokinesis) similar to that which occurs in tissue- 
 cells, except that the resulting daughter-cells are of different 
 sizes, two small nucleated polar-cells (Fig. 2) being successively 
 thrown off from the larger ovum, the portion of the original 
 nucleus remaining in the ovum being known as the "fern-ale 
 2)ronuclcus." A. sperm-cell (spermatozoon) then makes its way into 
 the ovum, and its nucleus (the male pronucleus) unites with the 
 female pronucleus to form the segmentation nucleus. This 
 process, which is known as impregnation or fertilisation, thus 
 consists in a material fusion of the generative substances of both sexes, 
 or more accurately of the sperm-nucleus and egg-nucleus. The essential 
 cause of inheritance can thus be traced to the molecular structure of 
 the nuclei of both male and female germinal cells. This structure 
 is the morphological expression of the characters of the species. 
 
 After fertilisation has taken place development begins. The 
 segmentation nucleus divides into two equal parts, each of which 
 forms a new centre for the division of the oosperm, as it must 
 now be called, into two halves or llastomeres. This division, the 
 beginning of the process of segmentation, takes place by the 
 formation of a furrow round the egg which becomes deeper and 
 deeper until the division is complete. (Fig. 2, A). 
 
 1 The vitellus consists of two different siibstances protoplasm and deutero- 
 jtiasm (yolk) in varying proportions in different animals. 
 
 B 2 
 
COMPARATIVE ANATOMY 
 
 The first stage in the process of segmentation is thus com- 
 pleted ; the second takes place in exactly the same way, and 
 results in a division of the oosperm into four parts, and by a similar 
 process are formed eight, then sixteen, then thirty-two blastomeres, 
 and so on, the cells becoming smaller and smaller, and each being pro- 
 vided with a nucleus (Fig. 2 C D). In short, out of the original 
 oosperm a mass of cells is formed which represents the building- 
 material of the animal body and which, from its likeness in appear- 
 ance to a mulberry, is spoken of as a morula. 
 
 In the interior of the morula a cavity (segmentation cavity or 
 
 C D 
 
 Fig. 2. DIAGRAMS OF THE SEGMENTATION OF THE OOSPERM. 
 
 A, first stage (two segments) : RK, polar cells. B, second stage (four segments). 
 C, further stage. D, morula stage. 
 
 blastoccele) filled with fluid is formed, and the morula is now spoken 
 of as the blastosphere or bias tula (Fig. 3). The peripheral cells 
 enclosing this cavity form the germinal membrane or blasto- 
 derm. Consisting at first of a single layer of cells, the blastoderm 
 later on becomes two- and then three-layered. From the relative 
 positions of these, they are spoken of respectively as the outer, 
 middle, and inner germinal layers, or as epiblast, (ectoderm,) 
 mesoblast, (mesoderm,) and hypoblast (endoderm). 
 
 An increase in the amount of food-yolk (deuteroplasm, see note on 
 p. 3) present in the ovum results in certain modifications of the primi- 
 tive process of segmentation as described above. Yolk is an inert 
 
INTRODUCTION 
 
 FIG. 3. BLASTOSPHERK. 
 
 substance, and its presence tends to hinder or even entirely to 
 prevent segmentation in those parts of the ovum in which it is 
 abundant. When the whole ovum undergoes division, the 
 segmentation is known as entire or holoblastic ; when division is 
 restricted to part of the ovum 
 only, the segmentation is said 
 to be partial or meroblastic 1 
 (Fig. 4). 
 
 The question as to the origin 
 of the germinal layers, on ac- 
 count of its important significa- 
 tion, is one of the most burning 
 problems in Morphology, and 
 as yet we cannot arrive at any 
 full and satisfactory conclusion 
 on the subject. It may, how- 
 ever, be affirmed with certainty 
 that in all Vertebrates the 
 
 blastosphere passes Or did SO BDt blastoderm ; FH, segmentation 
 
 in earlier times into a stage cavity. 
 
 called the gastrula. One 
 
 must imagine this form as being derived primitively from the 
 blastula by supposing that the walls of the latter (Fig. 3) became 
 pushed in or invaginated at one part, thus giving rise to a double- 
 
 walled sac (Fig. 5) The outer wall then 
 represents the epiblast, which functions 
 as an organ of protection and sensation, 
 while the inner, or hypoblast, encloses 
 a central space, the primitive intestinal 
 cavity (archenteron), and represents the 
 assimilating and digestive primary ali- 
 mentary canal. The opening of the 
 latter to the exterior, where the two 
 germinal layers are continuous, represents 
 FIG. 4. DIAGRAM OF A MER- the primitive mouth or blastopore 
 
 o* 5) 
 
 Q^ of the epiUat arige kter the 
 
 Bla, blastoderm; Do, yolk, epidermis and its derivatives, the entire 
 
 nervous system, the sensory cells, the 
 
 crystalline lens of the eye, and the oral and anal involutions 
 (stomodceum and proctodceuni). In an early stage the hypoblast 
 gives rise to an axial rod, the notocliord (see p. 9), and eventually 
 to the epithelium of the greater part of the alimentary canal 
 
 1 In holoblastic segmentation the resulting cells are approximately equal in 
 the Lancelet and in Mammals (with the exception of Monotremes) ; and unequal 
 in the Cyclostomes, Sturgeon, Lepidosteus, Ceratodus, and nearly all Amphibians, 
 the segmentation sometimes approaching the meroblastic type. In Elasmo- 
 branchs, Teleosts, Reptiles, Birds, and Monotremes the segmentation is meroblastic 
 and discoid, i.e., restricted to the upper pole of the ovum (Fig. 4). 
 
 ffla, 
 
 OBLASTIC OOSPERM WITH 
 
 DISCOID SEGMENTATION. 
 
6 COMPARATIVE ANATOMY 
 
 (Fig. 6, A and B) with its glands, including the thyroid, thymus, 
 liver and pancreas, as well as to the epithelial parts of the gill- 
 sacs and lungs. 
 
 Though we may look upon the epiblast and bypoblast, that 
 is, both the primary germinal layers as arising in the manner 
 above described, the problem as to the origin of the mesoblast is as 
 yet by no means settled. All that can be said at present is briefly 
 as follows : The mesoblast is a secondary formation, and is phylo- 
 genetically younger than the other two germinal layers; both 
 as regards the origin of its cells -and histologically, it is of a com- 
 pound nature, and thus forms a marked contrast to the germinal 
 layers proper. Reminding one in many points of the " mesenchyme " 
 of Invertebrates, it always arises at first from the point where 
 
 FIG. 5. GASTRULA. 
 Ekt-, epiblast ; Ent, hypoblast ; B1p t blastopore ; U, archenteron. 
 
 epiblast and hypoblast pass into one another, that is, from the 
 region of the blastopore, or, what comes to the same thing in the 
 higher Vertebrates, from the primitive streak. Originating from 
 between the other two layers, one of its first and most important 
 functions is the formation of blood-cells ; later it gives rise to the 
 heart, vessels, supporting and connecting substances (connective-tissue, 
 adipose tissue, cartilage, and bone), serous membranes (peritoneum, 
 pleura, pericardium, arachnoid), muscles, and almost the entire 
 excretory and reproductive apparatus. 
 
 A cleft appearing in the mesoblastic tissue divides it into a 
 parietal or somatic layer (Fig. 6, A and B), lying along the inner 
 side of the epiblast, and into a visceral or splanchnic layer, which 
 becomes attached to the outer side of the hypoblast. The former, 
 together with the epiblast to which it is united, constitutes the 
 
INTRODUCTION 
 
 JEnl 
 
 Ekt 
 
 FIG. 6, A AND B. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A DEVELOPING 
 VERTEBRATE EMBRYO. 
 
 JJ, alimentary canal ; Eat, hypoblast, showing in Fig. A the thickening (Ch) which 
 will form the notochord ; Ch l (Fig. B), the notochord now constricted off from 
 the hypoblast ; UW, mesoblastic somite ; UG, primary urinary duct (pro- 
 nephric duct); A, aorta; 8pP, splanchnic and Sop, somatic mesoblast ; 
 Co, Ca-I, ccelome ; H, remains of the upper part of the ccelome in the 
 interior of the mesoblastic somites ; Ekt, epiblast ; Med, central nervous 
 system (medullary cord) : in Fig. A it is shown still connected with the 
 epiblast, from which it has become constricted off in Fig. B. 
 
8 COMPARATIVE ANATOMY 
 
 somatopleure, and the latter, together with the hypoblast, the 
 splanchnopleure. The cavity separating these is the body cavity, 
 or coelome (Fig. 7), 1 and is lined by an epithelium. The dorsal 
 part of the mesoblast which lies on either side of the middle line 
 early becomes transversely segmented to form a series of mesoblastic 
 somites or protovertebrce, which lose their cavities (Fig. 6, A and B) 
 and are concerned in the formation of the vertebral column, body 
 muscles, and urinogenital apparatus. 
 
 As a general rule a thickened disc-shaped region can be recog- 
 nised at a certain stage of development on the dorsal pole of the 
 
 KW 
 
 FIG. 7. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE BODY or AN ADULT 
 
 VERTEBRATE. 
 
 Med, spinal cord ; JV7?, neural tube ; K W, body-wall ; Co, dermis ; Ep, endodermic 
 epithelium of alimentary canal (intestine) ; VR, visceral tube ; Ao, aorta ; 
 Ms, mesentery ; Per, parietal layer of the peritoneum ; Per 1 , visceral layer of 
 the peritoneum ; Mac, muscular coat of intestine ; Siibm, connective-tissue coat 
 of intestine ; DH, lumen of intestine ; W, vertebral centrum with dorsal arch. 
 
 oosperm : this is the so-called embryonic area, and on it the first 
 indications of the body are seen. This region gradually becomes 
 constricted off from the yolk by the formation of furrows at its 
 anterior and posterior ends as well as laterally, and consequently 
 the connection of the body-rudiment with the ventral yolk-sac (the 
 
 1 The coelome may arise as a segmentally arranged series of pouches 
 (enterocceles) from the archenteron, in which case its lining epithelium is at first 
 continuous with the hypoblast, as is most plainly seen in Amphioxus ; or it may 
 be formed secondarily by a splitting (delamination) of the mesoblastic tissue 
 (schizoccde). The first of these must be considered as the more primitive. 
 
INTRODUCTION 9 
 
 vitello-i'iitesti'iial duct) is reduced in size, and when the yolk is 
 eventually entirely absorbed, disappears altogether (Fig. 8, f). In 
 the higher Vertebrates (Reptiles, Birds, and Mammals) folds of the 
 soinatopleure arise externally to these furrows, and are known 
 respectively as the head, tail, and lateral folds ; these gradually 
 grow upwards and eventually unite with one another dorsally so 
 as to form a membranous, dome-like sac, the amnion (Fig. 8) 
 which encloses the embryo and contains a fluid (liquor amnii). 
 
 Owing to the presence of this structure the above-named 
 Vertebrates are usually distinguished as Amniota from the 
 Anamnia (Fishes and Amphibians), in which no amnion is 
 developed (p. 13). 
 
 A network of blood-vessels becomes developed over the yolk- 
 sac, which may therefore serve as an organ of respiration as 
 well as of nutrition. But in the higher Mammals this func- 
 tion is only a very subsidiary one, as at a very early stage a 
 vascular sac-like outgrowth, the allantois (Fig. 8), arises from 
 the hinder part of the intestine (i.e., from the splanchnopleure). 
 This serves not only for respiration, but also for the reception of 
 excretory matters derived from the primitive kidney. It is also 
 present in Amphibians, but in them remains small, and does not 
 extend beyond the body cavity of the embryo; while in the 
 Amniota it gradually increases in size and grows round the embryo 
 as a stalked vesicle, which in Reptiles, Birds, and Monotremes 
 conies to lie close beneath the egg-shell arid acts as an efficient 
 respiratory organ during the rest of the embryonic period. 
 Towards the close of this period the allantois gradually undergoes 
 more or less complete reduction. 
 
 In the higher Mammalia, however, an important vascular con- 
 nection takes place between the mother and fcetus by means of the 
 allantois. The latter becomes attached to a definite region of 
 the uterine wall, and from it vascular processes or mlli arise, so 
 that the foetal and maternal blood-vessels come into very close 
 relations with one another. Thus an allantoic placenta is 
 formed, which serves both for the respiration and nutrition of the 
 fcetus (Fig. 9). As an allantoic placenta is not developed in 
 Monotremes and is only slightly indicated amongst Marsupials, 
 these forms are distinguished as Aplacentalia from the higher 
 Mammals, or Flacentalia (p. 14). 
 
 The following important points must be noted as regards the 
 structure of the Vertebrate body. After the main organs have ap- 
 peared, a smaller dorsal neural tube and a larger ventral visceral 
 tube extend longitudinally through the body, and between the two 
 is a rod-like supporting structure, the noto chord (p. 5), which 
 arises as an axial thickening of the primary hypoblast and forms the 
 primitive skeletal axis : it is usually replaced by a vertebral column 
 consisting of centra and arches, at a later stage of development 
 (Fig. 7). All these are median in position, and the body is thus- 
 
10 
 
 COMPARATIVE ANATOMY 
 
 A1.-..I- 
 
 FIG. 8, A, B, AND C. DIAGRAMS ILLUSTRATING THE FORMATION OF THE AMNION, 
 ALLANTOIS, AND YOLK-SAC. A AND B, IN LONGITUDINAL SECTION ; C, IN 
 TRANSVERSE SECTION. 
 
 JK, embryo ; Dh, alimentary cavity ; Do, yolk-sac ; t, vitello-intestinal duct ; PP, 
 ccelorne ; Ah, amniotic cavity ; AF, amniotic fold ; A, amnion ; At, allantois ; 
 n, somatopleure ; b, splanchnopleure ; M, medullary cord ; C, notochord. 
 
INTRODUCTION 
 
 11 
 
 bilaterally symmetrical. The neural tube, or cerebro-spinal cavity, 
 
 enclosed by the skull and vertebral arches, contains the central ner- 
 vous system (brain and spinal cord] ; the visceral tube (ccelome, 
 p. 8, Fig. 7) encloses the viscera (alimentary canal, urinogenital 
 organs, &c.),and its muscular walls may be strengthened by a series 
 
 Pf(CLf) 
 
 FIG. 9. DIAGRAMMATIC SECTION THROUGH THE HUMAX GRAVID UTERUS. 
 
 U, uterus ; Tb, Tb, Fallopian tubes ; UH, uterine cavity ; Dv, clecidua vera, which 
 at Pu passes into the uterine portion of the placenta ; Dr, decidua reflexa ; 
 Pf, foetal portion of the placenta (chorion frondosum, Chf) ; Chi, chorion 
 laeve ; A, A, the cavity of the ainnioii filled with fluid : in the interior of the 
 amnioii is seen the embryo suspended by the twisted umbilical cord ; H, 
 neart ; A, aorta ; en, precaval, ci, postcaval, and p, portal vein ; At, allantoic 
 (umbilical) arteries ; t, the liver, perforated by the umbilical vein ; D, the 
 remains of the yolk-sac (umbilical vesicle). 
 
 of ribs, articulating dorsally with the vertebral column. Certain 
 of the ribs may reach the mid-ventral line and come into connec- 
 tion with a breast-bone or stermim, and thus form complete rings 
 or hoops around the visceral tube. 
 
 The anterior ends of the central nervous system (brain) and ali- 
 mentary tract enter into close relations with the outer world, the 
 
12 COMPARATIVE ANATOMY 
 
 former coming into connection with the higher sense-organs, while 
 from the latter are developed the mechanisms for the taking in of 
 nutriment and for respiration. 
 
 The anterior portion of the body, or head, passes behind into 
 the trunk, either with or without the intermediation of a neck. The 
 ccelome is practically restricted to the trunk, in the hinder part of 
 which the intestinal (anal) and urinogenital apertures are situated, 
 and posterior to which again is the tail. Head, trunk, and tail 
 constitute the body-axis, as distinguished from the limbs or 
 appendages, which arise from the trunk and of which there are 
 typically two pairs. 
 
INTRODUCTION 
 
 13 
 
 SYSTEMATIC ZOOLOGY. 
 
 On the ground of their relationship to one another, animals 
 have been classified into certain divisions and subdivisions, which 
 are designated as Classes, Orders, Suborders, Families, Genera, and 
 Species. 
 
 A general classification of the principal existing Vertebrate 
 groups is given in the following table. 
 
 A. Acrania. 
 
 Amphioxus (Lancelet). 
 
 B. Craniata. 
 
 / I. CYCLOSTOMATA (Suctorial Fishes). 
 
 1. Petromyzontidse (Lamprey). 
 
 2. Myxinoidse (Myxine, Bdellostoma). 
 
 II. GNATHOSTOMATA (Animals provided with jaws). 
 
 (a.) ANAMNIA (without amnion). 
 
 1. PISCES (True Fishes). 
 
 a. Elasmobranchii (Sharks and Rays). 
 
 3. Holocephali (Chimsera and Callorhynchus). 
 
 y. Ganoidei. 
 
 1. Selachoidei (Cartilaginous Ganoids Aci- 
 
 penser, Scaphirhynchus, Polyodon). 
 
 2. Teleostoidei (Bony Ganoids Polypterus, 
 
 Calamoichthys, Lepidosteus, Amia). 
 8. Teleostei. 
 
 1. Physostomi (with open pneumatic duct 
 
 between the air-bladder and pharynx, 
 e.g., Cyprinus, Salmo, Silurus, Mor- 
 myrus). 
 
 2. Physoclisti (air-bladder, when present, 
 
 with closed pneumatic duct, e.#.,Perca, 
 Gadus, Lophius, Labrus, Plectognathi, 
 Lophobranchii). 
 
 2. DIPNOI. 
 
 1. Monopneumones (Ceratodus). 
 
 2. Dipneumones (Protopterus, Lepidosiren). 
 
 3. AMPHIBIA. 
 
 a. Urodela. 
 
 1. Perennibranchiata (Proteus, Siren, 
 
 Necturus). 
 
 2. Caducibranchiata. 
 
 Derotremata (Amphiuma, Menopoma). 
 
 Myctodera (Salamandra, Triton, Am- 
 
 blystoma). 
 
 /3. Gymnophiona (Footless Csecilians). 
 y. Anura (Frogs and Toads). 
 
 Ichthyopsida. 
 
14 
 
 COMPARATIVE ANATOMY 
 
 Sauropsida. 
 
 Mammalia. 
 
 (b. A MX IOTA (Vertebrates which develop an aimiioii 
 during fetal life). 
 
 1. REPTILIA.. 
 
 a. Crocodilia (Crocodiles and Alligators). 
 
 #. Lacertilia (Lizards, including Hatteria). 
 
 y. Chelonia (Turtles and Tortoises). 
 
 8. Ophidia (Snakes). 
 
 2. AVES. 
 
 a. Ratitce (Cursorial Birds Ostrich, Rhea, Emu, tfcc.).- 
 0. CarinatcK (Birds of flight). 
 
 1. Aplacentalia or Achoria. 
 
 a. Prototheria or Ornithodelphia (Monotremata Orni- 
 
 thorhynchus and Echidna). 
 /3. Metatheria or Didelphia (Marsupialia Kangaroos,, 
 
 Phalangers, Opossums, &c.), 
 
 2. Placentalia or Choriata. 
 
 Eiitheria or Monodelphia. 
 Edentata. 
 Sirenia. 
 Cetacea. 
 Ungulata. 
 Hyracoidea. 
 Proboscidea. 
 Rodentia. 
 Cheiroptera. 
 Insectivora. 
 Carnivora. 
 Lemuroidea 
 Primates. 
 
INTRODUCTION 
 
 15 
 
 
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SPECIAL PART. 
 A. INTEGUMENT. 
 
 THE skin consists of a superficial ectodermal and a deeper 
 mesodermal layer. The former is called the epidermis (scarf- 
 skin) and the latter the dermis (corium, cutis). The subcutaneous 
 connective tissue is usually not sharply marked off from the dermis, 
 but the one passes gradually into the other. The epidermis always 
 consists of cells only, while the dermis is made up principally of 
 connective tissue fibres, and may also enclose muscular fibres. 
 Bony structures may occur in the dermis, as well as vessels 
 and glands, which only rarely extend into the epidermis, from 
 which the glands are all derived and with which they usually 
 remain in connection by means of their ducts. Nerves, migratory 
 leucocytes (lymph- or white blood-corpuscles), connective-tissue cells, 
 including pigment-cells (chromatophores) and free pigment, are found 
 in both layers of the integument. 
 
 Pigment is never formed in the epithelial or connective-tissue cells them- 
 selves, but always originates in the blood. 
 
 In the epidermis two layers may in general be distinguished : 
 a superficial, composed of flattened and hardened cells (stratum 
 corncum, horny layer), and a deeper layer made up of soft proto- 
 plasmic cells (stratum Malpighii, mucous layer). The latter serves 
 as a matrix for the regeneration of the horny layer, the superficial 
 part of which is continually scaling off. From the epidermis the 
 cuticular organs and integumentary glands, and all other parts 
 spoken of as epidermic structures take their origin. Such are, 
 hairs, bristles, nails, claws, hoofs, &c. The peripheral sensory end- 
 organs of the skin as well as the crystalline lens of the eye also arise 
 by a differentiation of epidermic cells (p. 5) : the definite relation 
 which many of these organs have with the dermis must be looked 
 upon as a secondary acquirement. 
 
 Amphioxus, Fishes, and Dipnoans. The surface of the 
 epidermis is covered with cilia in the larval Amphioxus (gastrula 
 stage), and this must undoubtedly be considered as inherited from 
 Invertebrate ancestors. The striated cuticular border of the outer 
 
INTEGUMENT 
 
 17 
 
 epidermic layer in many fishes (e.g. Cyclostomi, Teleostei, and 
 Dipnoi), and, as will be mentioned presently, in Amphibian larvae, 
 indicates the former possession of cilia (Figs 10 and 11). 
 ' Goblet-cells (unicellular glands) are very abundant in the many 
 layered epidermis of Cyclostomes (especially Myxinoids) and 
 osseous Fishes, and are extremely numerous in Protopterus. 
 
 Protopterus buries itself in the mud during the dry season, and its integu- 
 ment, which, besides the numerous goblet-cells, also contains simple multi- 
 cellular glandsli]\.e those of the Amphibia, gives rise to a varnish-like secretion 
 as well as to a hardened capsule or " coccoon," by means of which the animal 
 
 FIG. 10. DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE OF 
 THE SKIN IN FISHES. 
 
 Ep, epidermis ; Co, derma ; F, subcutaneous fat ; OS, cuticular margin ; Ko, 
 goblet-cells ; B, B, goblet-cells opening on the surface ; Ko, granular slime- 
 secreting cells present in Petromyzon and Malopterurus; G, vessels which 
 pass upwards in the vertical connective -tissue bundles of the derma ; W, hori- 
 zontal connective- tissue bundles. 
 
 is protected during its torpid period. In all Fishes which possess slime- 
 secreting cells in the integument, it is probable that the secretion serves to 
 protect the outer skin from the action of the water. 
 
 Midticelhdar glands are not commonly present in the integu- 
 
 c 
 
18 COMPARATIVE ANATOMY 
 
 ment of Fishes, but apart from Protopterus (see above) there are a 
 number of exceptions to this rule. 
 
 In male Elasmobranchs there is a large glandida pterygopodii (gland 
 of the clasper) at the base of each pelvic fin : it arises as a tube-like imagin- 
 ation of the skin, and is in relation with the copulatory organs. Poison-glands 
 are found amongst the Teleostei. Thus in the Weever (Trachinus) there 
 is a series of poison-glands lying on either side of the bases of the spines of 
 the dorsal fin and operculum. In Thalassophryne the operculum is provided 
 with a hollow spine, at the base of which a poison-gland is situated, and in 
 Synanceia there is also a series of glands at the bases of the grooved dorsal 
 spines. Poison organs are also present in Scorpsena and others ; but in many 
 cases in which such organs have been described a more detailed histological 
 examination is desirable. The phosphorescent and eye-like organs present in 
 the integument of some Fishes (Scopelidse, Chauliodus, &c.) are probably 
 to be looked upon as modified glands. 
 
 In Lepidosiren, apparently in the male only, the integument of the 
 pelvic fins is provided with numerous (? erectile) villi. 
 
 Pigment-cells, which are under the influence of the nervous 
 system and are able to cause a change of colour, are present some- 
 times in both layers of the integument, sometimes in the epidermis 
 only. The colouration is sometimes protective (e.g. Flat-fishes) and 
 sometimes sexual (e.g. Stickleback). 
 
 The bony scales of Fishes lie in connective-tissue pouches of 
 the dermis and are formed as ossifications of the latter. In 
 Teleosts and Dipnoans they are covered by the epidermis through- 
 out life ; in Ganoids and Elasmobranchs this is only the case in the 
 larva. In Teleosts the parts of the epidermis covering the ex- 
 ternally visible portions of the scales become cornified. (For 
 further details compare p. 30). 
 
 Amphibia. The epidermis of Amphibian larvse is for a short 
 period ciliated. In the adult, it may be said in general that the 
 
 FIG. 11. SKIX OF LARVA OF SALAMANDER (Salamandra, maculosa}. 
 
 Ep, epidermis ; Co, dermis ; a, stratum corneum ; ft, stratum Malpighii ; LZ, 
 Leydig's cells ; CS, striated border. 
 
 integument of Amphibians is intermediate in structure between 
 that of Fishes and Reptiles. 
 
 The epidermis of those larvse which live in the water consists 
 of two sharply differentiated layers. The outer layer is usually 
 made up of flat cells with a striated cuticular border on their 
 free edge (Fig. II), like that occurring amongst Fishes : the inner 
 
INTEGUMENT 19 
 
 layer is composed of more cylindrical or cubical cells. The former 
 corresponds to the stratum corneum, the latter to the stratum 
 Malpighii. The horny layer is shed periodically, either entire or 
 in pieces. 
 
 Later, with advancing development, the layers of the epidermis 
 become more numerous, and involutions towards the dermis take 
 place in all parts, giving rise to a great number of sac-like and 
 tube-shaped glands similar to those of Protopterus (p. 17); these 
 are particularly abundant in certain regions more especially on 
 the head and flanks (Fig. 12). The individual glands are sur- 
 
 FIG. 12. SECTION THROUGH THE SKIN OF ADULT SALAMANDER (S. maculosa). 
 
 Ep, epidermis ; Co, dermis, in the richly pigmented (Pi) connective-tissue stroma 
 of which the various sized integumentary glands (A , O, D, D, E) lie embedded ; 
 J/ 1 , the muscular layer of the glands, lying -within the basement membrane 
 (Pr) ; M, the same, seen from the surface ; E, epithelium of glands ; S, 
 secretion of glands ; Mm, subcutaneous layer of muscles, through which 
 vessels (G) extend towards the dermis. 
 
 rounded by muscle and connective-tissue fibres, pigment, blood- 
 vessels, and nerves. Their secretion serves to keep the skin moist, 
 but ns experiments have shown, it also forms an important weapon 
 of defence on account of its poisonous properties. 
 
 This richness in glands is a characteristic of the skin of Amphi- 
 bia and to it they owe their moist and slippery nature. Frequently, 
 as for instance in Toads, the skin is not smooth, but has a rough, 
 warty appearance, caused by local proliferations of the epidermis. 
 
 Epidermic claws, analogous to those of the Amniota, are present 
 only in Xenopus (Dactylethra) and Onychodactylus. 
 
 The pigment, accumulated principally in the dermis partly 
 diffused, partly enclosed within the cells is under the control of the 
 nervous system, and thus renders a change of colour possible ; and 
 
 c 2 
 
20 COMPARATIVE ANATOMY 
 
 as the colour becomes modified according to the surroundings of 
 the animal, it may serve as a protection (e.g. Hyla). 
 
 Calcifications may occur in the dermis, or, as in Ceratophrys 
 dorsata, definite bones may be formed (see p. 33) : the dermis also 
 encloses numerous smooth muscle-fibres. 
 
 Reptilia. The characteristic peculiarity of the skin of Reptiles 
 is its capacity of producing scales (these are very simple in Geckos 
 and Chameleons), warts, prickles, shields (e.g. the '' tortoiseshell " 
 of Chelonians), claws, rattles (Rattlesnake), and other epidermic 
 structures (Fig. 13). All these are due in the first instance to 
 the formation of dermal papilla?, the markedly stratified epidermis 
 covering which becomes cornified secondarily. The horny layer of 
 the epidermis may be periodically cast off either entire (Snakes) 
 
 FIG. 13. DIAGRAMMATIC SECTIONS THROUGH VARIOUS RINDS or EPIDERMIC 
 SCALES OF REPTILES. (From Boas's Zoology.} 
 
 A, rounded scales ; B, shields ; C, imbricating scales ; D, overlapping scales with 
 bony scutes in the underlying dermis; h, horny layer; *, Malpighian layer of 
 the epidermis ; /, dermis ; o, bony scutes. 
 
 or in shreds: it is renewed from the Malpighian layer. The 
 integument of Hatteria retains the most primitive characters 
 amongst Reptiles. 
 
 Pigment-cells occur in the integument, rendering a change of 
 colour possible in many cases (e.g. Chameleon). 
 
 Ossifications in the dermis are very common in Reptiles, and 
 there is great variation in the degree of their development, from 
 the small bony scutes present in Geckos (Ascalabota) to the large 
 exoskeletal plates of Chelonians (see p. '33). Muscles are also 
 present in the dermis. In contrast to the skin of Amphibians, 
 that of Reptiles is entirely wanting in glands. 
 
 In Lizards, the so-called femoral glands occurring along the ventral side 
 of the thigh are said to be merely solid cones of epidermic cells, which form 
 a series of papilla? or warts and serve as clasping organs during copulation. 
 
 Birds. Birds possess a thinner dermis than any other Ver- 
 tebrates, and it is less plentifully supplied with blood-vessels. 
 
INTEGUMENT 21 
 
 In the deeper layers there is a strongly developed network of 
 muscle-fibres showing traces of transverse striation : these are 
 inserted into the feather-sacs, and serve to erect the feathers. 
 
 Apart from a gland present in the neighbourhood of the 
 auditory passage amongst Gallinacese, there is only a single gland 
 situated at the base of the rudimentary tail (uropygium) : this 
 uropygial gland is present in nearly all Birds, and its secretion 
 serves to oil the feathers. Dermal bones are characteristically 
 absent, while epidermic structures, such as feathers, claws, spurs, 
 foot-scales, and beak-sheaths, are strongly developed. 
 
 One of the most marked characteristics of Birds is the pos- 
 session of feathers. In the majority of Birds they are of two 
 kinds down-feathers and contour-feathers, and are usually 
 arranged in so-called feather-tracts (pterylce) separated by naked 
 regions (apteria). The base of each feather is embedded in 
 an epidermic sac or follicle. Their mode of development corre- 
 sponds essentially with that of the epidermic scales of Keptiles. 
 
 In the region where a feather is to be formed, the dermal tissue becomes 
 raised up towards the ectoderm (Fig. 14, A), and thus gives rise to a vas- 
 cular papilla. As this papilla grows out to form an elongated cone with a 
 pointed apex, the feather-germ (B), its base sinks gradually deeper and deeper 
 into the dermis, and thus becomes surrounded by a sort of pocket the 
 feather-follicle. The horny, as well as the Malpighian layer of the epidermis 
 extends into the base of the follicle, and thence into the feather-germ, the 
 interior of which is throughout filled by cells of the dermis, which give rise to 
 the pulp. As the feather-germ keeps 011 growing, the cells of the Malpighian 
 layer begin to proliferate rapidly, giving rise to a series of radial folds 
 arranged along a central axis, which extend inward towards the pulp, and 
 are immediately bounded by the horny layer (C). These folds, between 
 which the nutritive pulp extends, then become cornified and separated from 
 above downwards from the surrounding cells ; and, by a gradual drying of 
 the central pulp-substance, give rise to a tuft of horny rays, which are, 
 however, at first bound together by the enclosing stratum corneum. Most 
 Birds are hatched when the feathers are in this stage of development, and 
 they thus appear as if covered with brush-like hairs. 
 
 By the shedding of the surrounding horny layer the rays or barbs become 
 free (D), and if these are all similar to one another, an embryonic down- 
 feather is formed. The whole feather-germ, however, does not become 
 divided up into barbs in this manner : its lower portion, embedded in the 
 skin, retains a more uniform character and forms the quill (calamus). 
 
 The embryonic down-feathers (E), on the individual barbs of which 
 smaller secondary rays or barbules become developed, may retain their char- 
 acter as such throughout life or may be replaced by definitive feathers. In 
 this case a second, larger, follicle early arises from the base of the follicle of 
 the down-feather, the pulp of the two being in connection (D). The papilla 
 developing within the interior of this new follicle grows rapidly, gradually 
 pushes the base of the down-feather out of its follicle, and comes to the 
 surface. 
 
 Each contour feather (penna) at first closely resembles a down- 
 feather (pluma) in structure, and consists of a tuft of similar rays 
 or barbs provided with secondary rays or barbules. In the course 
 of further growth, however, one of the rays becomes rapidly 
 
COMPARATIVE ANATOMY 
 
 thickened, and forms a main axis or stem (scapus), to which the 
 barbs are attached on either side. The proximal or basal portion of 
 the scapus which bears no barbs is called the quill (calamus), and 
 the distal part, to which the barbs are attached, the shaft (rachis). 
 The barbs together constitute the vane (vexillum') (Fig. 14, F). 
 
 f. 
 
 FIG. 14. Six STAGES ix THE DEVELOPMENT OF THE FEATHER. 
 ( Mainly after Th. Studer. ) 
 
 Cu, dermis ; SM, stratum Malpighii ; Sc, stratum corneum ; SM 1 , &V 1 , extensions 
 of these tissues into the feather-papilla, Pap ; FK, feather-germ ; F, F 1 , 
 feather-follicle ; P, pulp ; Fal (SM 1 ), folds of the Malpighian layer extending 
 into the feather-germ, and enclosed externally by the horny layer, HS (Sc 1 ) : 
 both layers are seen in the transverse section (C) ; FSp, quill of feather, which 
 breaks up above into a tuft of rays or barbs (HSt) ; sec, sec, secondary rays 
 (barbules) arising from the latter ; J?, rachis ; V, vexillum. 
 For further details as to the different stages A-F, compare text. 
 
 The barbules are so arranged on each barb as to make the latter 
 resemble an entire feather in appearance. The barbs may become 
 very closely united together by means of minute hooks on the 
 barbules, so that an extremely strong and resistant though pliant 
 structure is formed ; this is especially the case in the large wing 
 and tail feathers (remiges and redriccs). 
 
 In many Birds each quill of the ordinary feathers of the body bears two 
 vexilla, the second being spoken of as the aftereihaft (hyporctchis). 
 
INTEGUMENT 23 
 
 A periodic casting of feathers, or moulting, takes place in 
 :all Birds, and corresponds to the similar process of the casting of 
 the horny layer of the skin in Amphibians and Reptiles. 
 
 The feather-covering of Birds must have been acquired in very early 
 geological periods, for Archseopteryx, found in the Jurassic strata of Bavaria, 
 possessed well-formed feathers with a very delicate shaft and vane. Palseonto- 
 logical researches have not brought to light any definite intermediate stages 
 between scales and feathers, but that they must once have existed is shown 
 by the development of these structures. 
 
 Mammals. The integument of Mammals gives rise to hairs, 
 which are characteristic of and confined to this Class. They may be 
 almost uniformly developed all over the body and even on the soles 
 of the feet, or may become reduced in more or less extensive regions. 
 They are most scanty in the Cetacea, where only a few occur on 
 the lips, and even these may disappear in the adult. The 
 first to appear are certain tactile-hairs (vibrissce) on the head, along 
 the course of the trigeminal nerve ; all the hairs, however, serve 
 as tactile organs as well as for keeping the body warm. 
 
 Nothing definite can be at present stated as regards the 
 phylogeny of hairs, but it seems at any rate probable that they 
 are not directly comparable to the scales of Reptiles and feathers 
 of Birds : l the arrangement of the hairs in alternating groups is 
 probably the last indication of the former possession of scales. 
 
 Each hair first arises as a proliferation of the epidermic cells 
 in the region of the Malpighian layer which comes to project 
 inwards towards the derm is (Fig. 15, A, B and C). In this 
 manner the hair-germ is formed. Thus the epidermic portion is the 
 primary one ; a corresponding dermal papilla is formed secoiidarily, 
 and is the homologue of the papilla which forms the first trace of 
 the scale in a Reptile or the feather in a Bird. 
 
 The thickening of the epidermis then grows downwards in the form of a 
 papilla and becomes surrounded by the cells of the dermis, so that, as in the 
 case of the feather, it comes to lie within a kind of pocket, the hair-follicle 
 (Fig. 14, C). The originally uniform mass of cells of the hair-germ later 
 becomes differentiated into a peripheral and a central portion. The latter 
 consists of more elongated cells, and gives rise later to the hair-shaft with its 
 medulla or pith, and to the cortex, as well as to the cuticle of the shaft and 
 the so-called inner root-sheath; the former gives rise the outer root-sheath 
 (comp. Fig. 16 A, which represents the fully-formed hair). The base of the 
 hair-shaft which tills up the bottom of the follicle is broadened out to form 
 the hair-bulb (Fig. 15, D), which grows round the later formed and highly 
 vascular hair-papilla like a cap (C, D). At Dr, in D, the sebaceous glands 
 (p. 27) are seen arising by a proliferation of the Malpighian cells. The hair 
 usually breaks through the skin in an oblique direction ; the direction differ- 
 ing in different parts of the body. 
 
 The hair or hair- shaft embedded at its base in the hair- 
 follicle, is more or less cylindrical : it consists of three parts 
 
 1 It has been suggested that the hairs correspond to modified integumentary 
 sense-organs such as occur in the lower Vertebrates (comp. Figs. 15 and 150). 
 
24 COMPARATIVE ANATOMY 
 
 medulla, cortex, and cuticle (Fig. 16 A), all of which are formed 
 from cells. The follicular tissue, which is richly provided with 
 blood-vessels, extends into the bulb-like base or root of the hair- 
 
 HP 
 
 FIG. 15. DIAGRAMS OF FOUR STAGES (A-D) IN THE DEVELOPMENT OF HAIRS, 
 
 (After F. Maurer.) 
 
 Sc, stratum corneum ; SM, stratum Malpighii, which gives rise to an epithelial 
 knob at Ep ; this grows inwards into the dermis (C) ; F, rudiment of the hair- 
 follicle ; HP, hair-papilla ; HK, hair-bulb ; Dr, rudiment of the sebaceous 
 gland. In D, /, indicates the stratum lucidum with eleidm-granules in the 
 cells. 
 
 shaft, and gives rise to the hair-papilla. From this region a new 
 hair-shaft may develop when the hair is shed, periodically or n on- 
 period ically as the case may be, often by the formation of a new 
 papilla. The colour of the hair is due to three causes : Firstly, 
 
INTEGUMENT 25 
 
 to the greater or less accumulation of pigment in the cells of the 
 cortical layer ; secondly, to the air contained in the intercellular 
 
 FIG. 16, A. LONGITUDINAL SECTION THROUGH A HAIR. (Diagrammatic.) 
 
 Sc, stratum corneum ; SM, stratum Malpighii ; Co, dermis ; Ap, arrectores pili ; 
 Ft, adipose tissue ; F, outer longitudinal layer, and F 1 , inner transverse layer 
 of dermic coat (both composed of connective-tissue) ; Sch, hair-shaft ; J/, 
 medulla ; R, cortex ; 0, cuticle of shaft ; WS, WS\ external and internal 
 root-sheath, the latter reaches above only as far as the point of entrance of 
 the ducts of the sebaceous glands (HBD) ; HP, hair-papilla, containing 
 vessels ; GH, hyaline layer, which lies between the inner and outer hair- 
 sheaths, i.e., between the root-sheath and the follicle (dermic coat). 
 
 spaces of the medulla ; and lastly, to the nature of the surface of 
 the hair, i.e., whether it is rough or smooth. The hairs are usually 
 arranged in groups of finer and coarser elements, and, especially 
 in the case of the vibrissae, are well innervated. 
 
2G 
 
 COMPARATIVE ANATOMY 
 
 A richer hairy covering (lanugo) is often met with in the embryonic 
 condition as, for instance, in the human foetus than occurs later ; and this 
 fact, together with the occasional appearance of abnormally hairy individuals, 
 indicates that at one time Man was distinguished by a far more abundant 
 clothing of hair than at the present day. 
 
 Other epidermic structures, formed as thickenings of the horny 
 layer, also play a very important part in Mammals ; such are 
 daws, nails, bristles, and spines (Hedgehog, Porcupine) ; the so- 
 called whalebone (baleen) of the Mystaceti ; the horn-sheaths in 
 Ruminants ; the nasal horns of the Rhinoceros ; the scales in Manis 
 and on the tail of the Beaver and other Mammals ; the palatal 
 plates of Sirenia ; and the ischial callosities of certain Apes. 
 
 When pigment is present, as, for instance, on the snout in many 
 Mammals and on the external genitals (labia rnajora and scrotum) 
 and the teats in the human subject, it is always situated in cells 
 of the Malpighian layer. 
 
 The outer layer of the dermis, as may be seen by a glance at 
 Fig. 16, B, may be divided into an outer papillary and an inner 
 
 reticular portion. The pa- 
 pillae of the former are ac- 
 curately adapted to the 
 over-lying epidermis : some 
 of them contain blood- and 
 lymph-capillaries, and others, 
 nerves with tactile cor- 
 puscles. The latter, on the 
 other hand, becomes lost 
 without any sharp boundary 
 line in the sub-dermal con- 
 nective-tissue and in the 
 more or less strongly-de- 
 veloped fatty layer (panni- 
 culus adiposus). The pads 
 (tori) on the soles of the feet 
 of most Mammals are due to 
 large dermal papilla. 
 
 In addition to numer- 
 ous elastic fibres, smooth 
 muscle elements are distri- 
 buted throughout the der- 
 
 16, B. SECTION THROUGH THE HUMAN 
 SKIN. 
 
 .Sc, stratum corneum; SM, stratum Malpighii ; 
 Co, dermis ; F, F, subcutaneous fat ; NP, 
 sensory papillse ; GP, vascular papillae ; N 
 and G, nerves and vessels of the dermis ; 
 SD, SD, sweat-glands, with their ducts 
 (SD 1 , SD 1 ) ; H, hair with sebaceous 
 glands (D). 
 
 mis ; they are particularly 
 abundant in the scrotum 
 (dartos) and in the teats, 
 and are present in connec- 
 tion with the hair-sacs 
 
 (arrectores pili) : the power of erecting the hair possessed by many 
 Mammals is due to these (Fig. 16, A). A bony dermal skeleton 
 is found only in the Armadillo amongst existing Mammals 
 (comp. p. 34). 
 
INTEGUMENT 
 
 27 
 
 The integumentaiy glands, which are well developed in all 
 Mammals except the Cetacea, are of two kinds, tubular and acinous. 
 The former include the siveat-glands and their various modifica- 
 tions ; while the latter are spoken of as sebaceous glands, and include 
 the already-mentioned glands of the hair-sacs, which serve to oil the 
 hair (Figs. 15 D, and 16, A and B), the preputial glands, the inguinal 
 glands of certain Rodents, the Meibomian glands of the eyelids, and 
 many others. It must be borne in mind, however, that there is 
 not always a sharp distinction between these two kinds of glands. 
 
 The paired femoral gland of Ornithorhynchus opens by means of a long 
 duct on to the spur present on the hind foot. Its secretion is poisonous. 
 
 Another important modification of the integumentary glands 
 of the Mammalia is seen in the mammary glands, which secrete 
 
 A B 
 
 g.m 
 
 FIG. 17, A. A, VENTRAL VIEW OF A BROODING FEMALE OF Echidna hystrix. 
 B, DISSECTION OF THE VENTRAL INTEGUMENT FROM THE DORSAL (INNER) 
 SIDE. (After W. Haacke.) 
 
 t, : f, the two tufts of hair in the lateral folds of the mammary pouch (b,m.) from 
 which the secretion flows. On either side of the pouch, which is surrounded 
 by* strong muscles, a group of mammary glands (g.m.) opens ; cl, cloaca. 
 
 milk for the nutrition of the young. In Monotremes these 
 apparently correspond to sweat glands, while in other Mammals 
 they represent sebaceous glands. 
 
 g. m. 
 
28 
 
 COMPARATIVE ANATOMY 
 
 Monotremes possess no teats, and the milk probably passes 
 along the hairs, which in this region are arranged in bunches, and 
 is then licked off by the young animal. The gland is compressed by 
 a strong sphincter muscle. In Echidna, a mammary or marsupial 
 pouch which is primarily paired and becomes unpaired secondarily, is 
 early formed for the reception of the young, and the gland-masses 
 open into two depressions of the ventral integument where the 
 bunches of hair are situated (Fig. 17). These depressions may be 
 called mammary pockets, and are of especial interest as they repre- 
 sent the first stage in the development of the various forms of teats 
 present in all other Mammalia, in many of which distinct indica- 
 tions of the Monotreme 
 condition are met with. 
 The marsupial pouch 
 of the Marsupialia i& 
 probably homologus 
 with that of Echidna. 
 
 Thus a similar 
 mammary pocket is 
 formed in the embryos 
 of Marsupials and pla- 
 cental Mammals by 
 the epidermis extend- 
 ing inwards towards 
 the dermis, and cylin- 
 drical, more or less 
 branched processes a- 
 rising from the base 
 of the pocket thus 
 formed (Fig. 17, B r 
 A). These processes 
 only are the glands 
 proper, the mammary 
 pocket being simply a 
 part of the outer sur- 
 face of the skin which 
 has sunk inwards, and 
 thus it may give rise to 
 hairs and other integu- 
 mentary structures. 
 
 The teats may be- 
 come developed in one 
 of two ways. In 
 
 the first of these, the skin surrounding the pouch becomes 
 raised up to form a circular rampart, and thus gives rise to a teat 
 perforated by a canal, into the base of which the ducts of the 
 gland open (Fig. 17, B, -#). In the second case, the gland sur- 
 face itself becomes elevated into a papilla, while the surrounding 
 
 FIG. 17, B. DIAGRAMMATIC REPRESENTATIONS OK 
 THE EARLY DEVELOPMENT OF THE LEADING 
 TYPES OF MAMMARY GLANDS. (Modified from 
 Gegenbaur. ) 
 
 A, first or undifferentiated (mammary pit) stage; 
 B, stage of the false teat ; (7, stage of the true 
 teat ; v, v, rim (or rampart) of the glandular 
 area ; /, .7, glandular area ; <//, mammary glands ; 
 d, mammary canal. 
 
INTEGUMENT 29 
 
 skin remains almost on a level with the rest of the integument 
 ((7). In the latter case the teats may be described as true or 
 secondary (Marsupials, Kodents, Lemurs, Monkeys, and Man), and 
 in the former as pseudo- or iwimary teats (Carnivora, Pigs, Horses, 
 and E-uminants). The latter condition is already indicated in 
 certain Marsupials (e.g. Phalangista vulpina). 
 
 The number of teats varies greatly : there may be as few as 
 one pair, or as many as eleven pairs (Centetes). They are often 
 situated in two nearly parallel rows along the ventral side of the 
 thorax and abdomen which slightly converge towards the inguinal 
 region : in other cases they may be restricted either to the inguinal 
 (Ungulates and Cetaceans) or to the thoracic region (Sloths, 
 Elephants, Sirenia, many Lemurs, Cheiroptera, and Primates) : 
 while in others, they may be axillary or abdominal, or they may 
 occur in various combinations of all these regions. 
 
 In the male, the mammary apparatus becomes aborted, though usually at 
 birth and puberty milk is produced in the human subject. Male goats and 
 castrated sheep have also been known to give milk, and the same is probably 
 true of male Bats. The occasional existence in men of supernumerary teats, 
 and in women of supernumerary mammae and teats (polymastism and poly- 
 fhelism) is very remarkable. They are usually situated in the thoracic region, 
 and must be considered as atavistic to a primitive form which possessed 
 numerous teats and which produced a number of young at a time. Such a 
 transition from polymastism to bimastism may be seen plainly at the present 
 day in the Lemurs : in them the inguinal and abdominal teats are seen in 
 various degrees of retrogressive metamorphosis, while a single pair of thoracic 
 teats remain well developed. This accords with the fact that most Lemurs 
 bear only a pair of young ones at a time, which they carry with them at the 
 breast. Moreover, in various Mammals a greater number of teats are present 
 in the embryo than in the adult. 
 
 The mammary glands, which are at first solid, become secondarily 
 hollowed out and further differentiated. The whole intermediate 
 tissue during lactation is filled with white blood-corpuscles 
 (leucocytes) ; and possibly the well-known structural elements of 
 milk, known as colostrums and milk-spheres, owe their origin to 
 these corpuscles, which have passed through the walls of the acini. 
 
B. SKELETON. 
 
 1. EXOSKELETON. 
 
 THE hard exoskeleton, consisting of bone or other calcified tissues? 
 must be distinguished from the horny exoskeletal parts described 
 in the last chapter, in which, however, the former was also referred 
 to. Thus it will be remembered that the term " scale " is some- 
 times used for a horny epidermal structure, and sometimes for a 
 bony dermal one (pp. 18, 20). 
 
 The first and most primitive hard structures in Vertebrates are 
 met with amongst Elasmobranchs in the form of small, pointed 
 
 denticles (placoid organs') in the skin ; 
 these consist of enamel and dentine, 
 resting on a basal plate of bone, thus 
 resembling in structure ordinary oral 
 teeth, which will be described later. 
 Primitively, as in many Rays, there is 
 a relatively small number of these 
 placoids, which do not touch one 
 another, while in most Sharks arid Dog- 
 fishes they are much more numerous 
 and close-set (Fig. 18). Their shape 
 is rhombic or more or less rounded, 
 each bearing a spine, and new ones 
 being continually formed. The enamel, 
 developed in connection with epidermic 
 cells, is the primary part of the den- 
 ticle (Fig. 19) ; the dentine is developed 
 secondarily that is, later from the 
 phyletically younger mesoderm, and 
 this is also true of the bony portion, 
 The enamel is therefore the first, and 
 originally the only hard substance of 
 t he placoid organ. 
 
 e first bony tissue to be developed 
 atomy.) is thus formed in connection with 
 
 FIG. 18. DERMAL DENTICLES 
 OF Centrophorus calcem, 
 
 An- 
 
EXOSKELETON 
 
 31 
 
 these denticles, the basal-plate representing an accessory portion 
 of the denticle, and serving to fix it within the skin. In the 
 further course of evolution the denticle itself undergoes reduc- 
 tion, the basal-plate remaining as an independent structure. 
 This is illustrated by a study of the exoskeleton in other 
 Vertebrates. 
 
 In the Holocephali dermal denticles are only present on 
 certain appendages (the claspers), and the first dorsal-fin is 
 strengthened by a large bony spine. 
 
 In most Ganoids thick plates, usually rhombic in form, are 
 present in the skin ; in bony Ganoids these cover the entire 
 body, their margins being in apposition. 1 These ganoid-scales- 
 correspond to the main (deeper) part of the placoid basal-plates, 
 
 FIG. 19. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK. 
 (From Gegenbaur's Comp. Anatomy.) 
 
 C, derails ; c, c, c, d, layers of the dermis ; p, papilla ; E, epidermis ; e, its layer 
 of columnar cells ; o, enamel layer. 
 
 the spine having become rudimentary. Their surface is dense 
 and smooth (ganoin-layer), and was formerly erroneously supposed 
 to consist of enamel. In Lepidosteus they bear numerous small 
 denticles ; but from what has been said above, this fact does 
 not indicate that each ganoid-scale corresponds to a multiple 
 of placoids. The exoskeleton was largely developed amongst fossil 
 Ganoids. 
 
 The scales of Teleosts, the first indication of which, as in 
 the case of placoid scales, is seen in the form of small papillae of 
 the dermis extending into the epidermis, correspond to the super- 
 ficial portions of the basal-plates. In the further course of develop- 
 ment they are seen to consist of bony plates arranged in oblique 
 rows and lying directly beneath the epidermis, the individual scales 
 not touching one another, and their surfaces lying parallel to the 
 
 1 In Amia, the scales have a " cycloid" form. (See note on p. 32.) 
 
32 COMPARATIVE ANATOMY 
 
 surface of the body. In this stage their arrangement resembles that 
 seen in Ganoids. Subsequently they usually come to lie within 
 definite pockets or sacs, and to overlap one another like tiles on a 
 roof (Fig. 20 A). The surface of the scales may be sculptured. 1 
 
 FIG. 20A. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE SKIN OF A 
 TELEOSTEAN, TO SHOW THE RELATION OF THE BONY SCALES. (Fro .11 Boas' s 
 
 Zoology. ) 
 
 /, dermis ; s, scale ; o, epidermis. 
 
 Amongst the Siluridse (Fig. 20, B), Plectognathi, and Lopho- 
 branchii, they may be of relatively large size and so arranged as to 
 form a strong bony cuirass. 
 
 Scales are wanting in Cyclostomes, and may be reduced or absent in repre- 
 sentatives of the three larger Orders described above (viz., in Electric Fishes 
 Spatularia, and some Eels). 
 
 In the Dipnoi the arrangement of the scales is similar to that 
 seen in the Teleostei. They consist of an external hard substance 
 
 FIG. 20u. DERMAL ARMATURE OF Callichthys. 
 
 B, barbules ; BrF, pectoral fin ; BF, pelvic fin ; RF, dorsal fin ; DS and VS, 
 dorsal and ventral bony shields; t, lateral line. 
 
 arranged in a network and provided with numerous denticles, and 
 of an internal portion composed of firm connective-tissue and bone. 
 
 1 Different forms of the rounded or polygonal scales in Teleostei are dis- 
 tinguished as cycloid and ctenoid. The former, which are the more primitive, 
 have a smooth margin, while in the latter the posterior margin is toothed and 
 comb-like. Various intermediate stages exist between the two forms. 
 
EXOSKELETON 
 
 33 
 
 These denticles are developed from connective-tissue cells, and are 
 not comparable to the placoid denticle ; the resemblance, too, between 
 the scales of the Dipnoi and Teleostei is only a superficial one. 
 
 Thus the exoskeleton plays an important part in Fishes, and 
 in numerous fossil Amphibians it reached a still higher develop- 
 ment (Stegocephala). Amongst these, specially strong dermal 
 plates were formed in the region of the shoulder-girdle, and very 
 commonly most of the body was covered with scales. Fossil genera 
 of Amphibia have, however, bequeathed but slight traces of this 
 strong dermal armour to the existing forms of the group: as 
 examples may be mentioned the bony plates in the skin of the 
 back of certain Anura (Ceratophrys dorsata and Ephippifer auran- 
 tiacus), as well as the scales lying between the ring-like scutes of 
 
 FIG. 21. A, CARAPACE, and B, PLASTRON OF A YOUNG TESTUDO GR^ECA ; C, 
 PLASTRON OF CHELONE MIDAS. 
 
 XX, neural plates ; (7, C, costal plates ; MM, marginal plates ; Np, nuchal 
 plate ; Py, Py, pygal plates ; E, entoplastron ; Ep, epiplastron ; Hy, hyoplas- 
 tron ; Hp, hypoplastron ; Xi, xiphiplastron ; #, R, ribs. ( V indicates the 
 anterior, and H the posterior end. ) 
 
 the footless Amphibia (Gymnophiona). The latter resemble in 
 many points the scales of Fishes and Dipnoans, and maybe derived 
 from such a scaly covering as that of the Permian Salamander, Dis- 
 cosaurus. 
 
 The dermal skeleton was still more highly developed amongst 
 fossil Reptiles, e.g., many Ornithoscelida (Stegosaurus). In these, 
 enormous bony plates and spines, sometimes as much as sixty-three 
 -centimetres long, were present in the dorsal region. Teleosaurus 
 also, as well as the Triassic Aetosaurus ferratus and the Cretaceous 
 Nodosaurus textilis, possessed a strong exoskeleton. Amongst 
 existing Reptiles (comp. p. 20), Crocodiles, many Lizards 
 (Anguis, Cyclodus, Scincus), and more especially Chelonians, 
 exhibit a well-developed dermal skeleton. In the latter Order 
 
 T) 
 
3.4 COMPARATIVE ANATOMY 
 
 there is a dorsal and a ventral shield (carapace &nd plastron) con- 
 sisting of numerous pieces and completely enclosing the body 
 (Fig. 21). Both arise independently of the endoskeleton, which 
 is preformed in cartilage : that is to say, they are true exo- 
 skeletal membrane bones (cp. note on p. 71). The exoskeleton, 
 however, comes into the closest relation with the endoskeleton, 
 and may supplant it here and there : thus, in Testudo, for 
 instance, the thoracic and lumbar vertebrse and ribs become quite- 
 rudimentary. 
 
 Birds, as already mentioned in the chapter on the integu- 
 ment, have no dermal skeleton, and this is true of all Mammals 
 except Armadillos (Dasypodidse). In these it consists of a serie& 
 of movable transverse bony scutes covering the head and body 
 and of smaller plates on the tail and limbs. Sparse hairs 
 occur between these plates. It is very doubtful whether this 
 exoskeleton has been derived from that of Reptiles : more 
 probably it, like the horny exoskeleton of Manis (p. 26), has 
 arisen secondarily, and in consequence of its development the 
 hairs have become reduced. In Glyptodon, a fossil member 
 of this group, the dermal plates were firmly united together to- 
 form a large shield which covered the whole body. 
 
 2. ENDOSKELETON. 
 
 I. VERTEBRAL COLUMN. 
 
 An elastic rod, the notochord or chorda dorsalis, lying 
 in the longitudinal axis of the embryo between the neural and 
 visceral tubes (see p. 9), is the first part of the endoskeleton 
 to be formed, and is the fore-runner of the vertebral column. It 
 is developed as a ridge of the primitive hypoblast, from which it 
 becomes constricted off, and is therefore of epithelial origin. The 
 large parenchyma-like cells of which it is composed consequently 
 do not give rise to any intercellular substance ; vacuoles, however, 
 soon appear within the cells, the protoplasm of which undergoes 
 modification, and thus a retrogressive metamorphosis sets in 
 (Fig. 22). The fact that this occurs at such early stages of develop- 
 ment shows that the notochord must long ago have begun to lose 
 its primitive function, whatever that function may have been. 
 
 As, these degenerative processes are gradually carried still 
 further, only the walls of the cells persist in the greater part of the 
 notochord ; these become flattened by mutual pressure, so that they 
 appear like a meshwork of pith-cells. At the periphery, however, 
 the cells retain their protoplasm, and become arranged like 
 an epithelium. Around the notochord two sheaths (Fig. 22 A, 1>) 
 
VERTEBRAL COLUMN 
 
 35 
 
 B 
 
 sk.l 
 
 FIG. 22. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE NOTOCHORDAL 
 SHEATHS AND VERTEBRAL COLUMN. 
 
 A. Early stage, showing notochordal cells (nc) and primary sheath (ah 1 ), as well 
 as the mesoblastic skeletogenous layer (sk.l). 
 
 B. Later stage, in which the central notochordal cells (nc) have become 
 vacuolated and the peripheral cells have given rise to the " notochordal epithe- 
 lium" (no. ep.) from which the fibrillar secondary sheath (h?) is derived: 
 paired dorsal and ventral cartilages (d.a, i.a) have arisen in the skeletogenous 
 layer. 
 
 C. Cartilage cells have passed through the primary sheath, and are invading 
 the secondary sheath (Cartilaginous Ganoids, Holocephali, Dipnoi, Elasmo- 
 branchii : in the last named chorda-centra are thus formed). 
 
 D. The cartilages are growing round the notochord, outside its sheaths, which 
 gradually become reduced : thus arch-centra are formed (Bony Ganoids, 
 Teleostei, Amphibia, Amniota). 
 
 A D represent the caudal region. 
 
 E. A later stage in the development of a pre-caudal vertebra. The notochord 
 (nc) has become constricted, and the cartilages have united into a single mass 
 and have given rise to a centrum (<), neural arch (n.a), neural spine (n. *p), 
 transverse processes (tr.j)) and articular processes (art). 
 
 D 2 
 
36 COMPARATIVE ANATOMY 
 
 are successively developed from its cells, and these differ both 
 chemically and physically from one another. The primary sheath 
 (so-called elastica) is first secreted by the peripheral notochordal 
 cells : the secondary sheath, which has a similar origin from 
 the so-called " notochordal epithelium," appears later, and occurs 
 in all the Craniata ; it is said not to be present in Ainphioxus, 
 the notochord of which, like that of the Tunicata, apparently 
 represents the oldest and most primitive form of this struc- 
 ture, such as is still repeated ontogenetically in Elasnaob ranch s. 
 The thick secondary sheath, which like the primary, is at first 
 homogeneous, gradually becomes fibrillar and replaces the primary 
 sheath functionally. 
 
 From the surrounding mesoblast a skeletogenous layer is de- 
 veloped : this not only surrounds the notochord, but extends 
 dorsally to it as well as ventrally (Fig. 22). Thus a continuous 
 tube of embryonic connective-tissue is formed enclosing the spinal 
 cord and only broken through at the points of exit of the spinal 
 nerves. This stage is known as the membranous stage, and in 
 it no indication is seen of the segmentation which occurs later in 
 the vertebral axis. The cause of this segmentation is to be traced 
 primarily to the muscular-system ; and it is evident, for mechanical 
 reasons, that the segmentation of the vertebral column must 
 alternate with that of the muscular segments or myotomes. Small 
 masses of cartilage arranged metamerically later appear in the 
 skeletogenous tissue close to the notochord, and these represent the 
 rudiments of the dorsal and ventral arches and bodies or centra of 
 the vertebra: (Fig. 22, B, D, E). This is the beginning of the 
 second or cartilaginous stage of the vertebral column ; the various 
 processes (spinous, transverse, articular, &c., Fig. 22, E) are then 
 formed, and now ossification may occur (bony stage). Those 
 parts of the fibrous tissue which do not become consolidated in this 
 manner give rise to the ligaments of the vertebral column. 
 
 During these differentiations of the skeletogenous tissue, the 
 notochord suffers a very different fate in the various Vertebrate 
 groups ; it may increase in size and persist as a regular cylindrical 
 rod, or it may become constricted at definite intervals by the forma- 
 tion of vertebral bodies, or even entirely disappear. 
 
 All these ontogenetic stages find their exact parallel in the 
 phylogenetic development of Vertebrates, as the following pages 
 will show. 
 
 Amphioxus, as already mentioned, apparently possesses the 
 most embryonic type of notochord. It is surrounded by a connec- 
 tive-tissue layer and is entirely unsegmented. 
 
 In Cyclostomes a very similar primitive condition is retained ; 
 but a secondary sheath becomes developed, and cartilaginous ele- 
 ments appear in the caudal region : in the adult Petromyzon 
 these are present all along the notochord in the form of rudi- 
 
VERTEBRAL COLUMN 37 
 
 mentary neural (dorsal) arches, which, however, do not meet above 
 the spinal cord. These cartilages, of which there are two pairs to 
 each muscular segment or myotome, correspond to the " intercalary 
 pieces" of Elasmobranchs (p. 38); they serve in the first instance 
 for the origin and insertion of the muscles, and at the same time 
 form a protection for the spinal cord. Neural spines also occur 
 in the middle of the axis, and in the caudal region hcemal 
 (ventral) arches enclosing the caudal aorta and vein, as well as 
 hcemal spines, are present, and fusion of the cartilaginous elements 
 occurs. 
 
 To the condition found in Cyclostomes, that seen in the 
 Cartilaginous Ganoids, Holocephali, and Dipnoi is directly 
 connected, inasmuch as the metameric character of the skeletal axis 
 
 FIG. 23. PORTION OF THE VERTEBRAL COLUMN OF Spatularia. (Side view.) 
 
 FIG. 24. TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF 
 ruthenu* (in the anterior part of the body). 
 
 Ps, spinous process ; EL, longitudinal elastic band ; SS, skeletogenous layer ; Ob, 
 upper arch ; M, spinal cord ; P, pia mater ; /c, intercalary pieces ; C, noto- 
 chord ; Ee, primary, and C$, secondary sheath of the notochord ; Ub, lower 
 arch ; Ao, aorta ; Fo, median parts of the lower arches, which here enclose 
 the aorta ventrally ; Z, basal processes of the lower arches. 
 
 is essentially indicated by the neural arches. In the two groups last 
 mentioned, however, skeletogenous cells break through the primary 
 notochordal sheath (elastica) and so invade the thick secondary 
 sheath, which in consequence encloses cartilage cells amongst its 
 fibres. In Chimsera calcified rings are also developed in the central 
 part of the sheath : these are more numerous than the arches. 
 The latter are developed as paired dorsal and ventral cartilages : 
 they remain cartilaginous in the Cartilaginous Ganoids (Figs. 23 
 and 24) and Holocephali, but become densely ossified in the Dipnoi 
 (Fig. 25). In the caudal region the haemal arches enclose the 
 caudal aorta and vein ; farther forwards the cartilages do not meet 
 in the middle line below, and consequently the lower arches end 
 
38 
 
 COMPARATIVE ANATOMY 
 
 on either side in a laterally-directed cartilaginous projection, or 
 basal process. 
 
 The relations of the arches in Elasmobranchs, Bony Ganoids 
 and Teleosts is similar to that above described. For the further 
 strengthening of the vertebral column so-called intercalary pieces 
 (Figs. 23, 24, 26, 28) appear between the upper and -lower arches 
 in Cartilaginous ! Ganoids and Elasmobraiichs, and these in the 
 
 C 
 
 FIG. 25. PORTION or THE VERTEBRAL COLUMN OF Protopterus. 
 C, notochord ; DF, neural spine ; FT, interspinous bone ; FS, fin-ray. 
 
 case of the dorsal arches are often spoken of as intcrneural plates. 
 In Elasmobranchs the neural arch may be made up of several 
 more or less distinct pieces the neural processes arising from the 
 centrum, the neural and interneural plates, and the neural spines. 
 
 In the Elasmobranchii, the skeletogenous cells invade the 
 notochordal sheath, as in the Holocephali and Dipnoi ; but the 
 sheath then becomes segmented to form a series of cartilaginous 
 
 Ob 
 
 im. 
 
 FIG. 26. PORTION OF THE VERTEBRAL COLUMN OF Scymnw. 
 
 WK, centra ; Ob, upper arches ; It; intercalary pieces. The apertures for^ the 
 spinal rerves are seen in the arches and intercalary pieces. 
 
 vertebral bodies or centra, which from the mode of their formation 
 may be called chorda-centra. The fact is thus accounted for that 
 the number of arch -elements does not necessarily correspond with 
 that of the centra in these Fishes. Ossification may occur in the 
 concave ends of the centra and in longitudinal bars along each 
 centrum. 
 
VERTEBRAL COLUMN 
 
 39 
 
 In Bony Ganoids and Teleosts paired dorsal and ventral carti- 
 lages likewise arise above and below the notochordal sheath, but 
 in the course of development so 
 extend at the base as to completely 
 surround it. From the dorsal carti- 
 lages the upper arches take their 
 origin, and from the ventral the 
 lower ; while the cartilage surround- 
 ing the notochord gives rise to the 
 vertebral centra, which may there- 
 fore be distinguished from those 
 described above as arch-centra. 
 
 In the development of the 
 centra of both kinds, the notochord 
 becomes constricted by the growth 
 of the cartilage at regular intervals, 
 while the latter undergoes segmen- 
 tation into centra. Each point of 
 
 constriction corresponds to the middle of a centrum, i.e., it is intra- 
 vertebral in position, and the notochord may here disappear entirely ; 
 
 WJC 
 
 FIG. 27. PORTION or THE VERTE- 
 BRAL COLUMN OF Polypterus. 
 
 WK, centra ; BF, basal processes ; 
 Ob, upper arches ; Pe t neural 
 spine. 
 
 c.n 
 
 . 28. PORTION OF THE VERTEBRAL COLUMN OF Lepidosteus. (After Balfour 
 
 and Parker. ) 
 
 vertebra" from anterior surface ; B, two vertebrae from the side, en, anterior 
 convex face, and o? 1 , posterior concave face of centrum ; h.a, basal process ; 
 ..a, upper arch ; i.c, intercalary cartilages ; fj, longitudinal ligament ; i.ft, 
 interspinous bone. 
 
40 COMPARATIVE ANATOMY 
 
 intervertelrally it remains expanded and so persists as a kind of 
 connecting- or packing-substance between contiguous centra, which 
 are consequently of a deeply biconcave or ampliiccelous form (Figs. 
 29A and 29s). 
 
 One of the Bony Ganoids, Lepidosteus, forms a marked excep- 
 tion to other Fishes as regards its vertebral column, inasmuch a& 
 definite articulations are formed between the centra, A con- 
 cavity is formed at the hinder end of each centrum (Fig. 28), which 
 articulates with a convexity on the next vertebra behind 
 (opistJioccelous form). The notochord (except in the caudal region) 
 entirely disappears in the adult ; in the larva it is seen to be ex- 
 panded intravertebrally, and constricted intervertebrally, a condition 
 of things which appears again in the higher types as, for instance^ 
 
 FIG. 29A. DIAGRAM SHOWING THE INTER VERTEBRAL REMAINS OF THE 
 NOTOCHORD. 
 
 C, O 1 , expanded and constricted portions of notochord ; WK, centra ; Li, intei - 
 
 vertebral ligaments. 
 
 FIG. 29u. PORTION OF THE VERTEBRAL COLUMN OF A YOUNG DOGFISH 
 (Scyllium canicula). After Cartier. 
 
 notochord ; Kn, outer, and Kn l , inner, zone of cartilage ; FK, the fibre-carti- 
 laginous mass lying between these zones, which is undergoing calcification ; 
 Li, invertebral ligament. 
 
 in Reptiles. In a still earlier larval stage, however, the constric- 
 tions are intravertebral, as in other Fishes. 
 
 The vertebral column of Fishes is characterised by a very 
 uniform character of its elements, so that a distinction can only be 
 seen between the trunk and caudal vertebrae. Its primitive 
 character is shown by the fact that the neural arches are usually 
 incomplete dorsally. As a rule, the closing in of the arch is 
 effected by special pieces of cartilage (comp. p. 38) and by an 
 elastic longitudinal band (Figs. 24, 28) which is always present : 
 this also applies to the haemal arches. Articular processes between 
 the arches (zyyapophyses) are usually present in Fishes which 
 possess bony vertebrae; in Rays and Chimaeroids only amongst 
 Fishes are definite articulations formed between the skull and 
 
VERTEBRAL COLUMN 
 
 41 
 
 vertebral column, and in these Fishes the anterior vertebra are 
 fused into a single mass. 
 
 In the caudal region of Amia the centra are mostly double, an archless 
 pleuro- or post-centrum alternating with an inter- or pre-centrnm. A some- 
 what similar condition is found in the Jurassic Eurycormus and other fossil 
 Ganoids. 
 
 As a rule Elasmobraiichs and Ganoids possess a greater number of 
 vertebrae (in Alopecias vulpes there are 365) than Teleosts, in which we 
 seldom meet with more than 70 : the Eel, however, possesses more than 
 200. 
 
 The caudal region of the vertebral column deserves particular 
 attention in Fishes, and the condition of this region in Amphioxus, 
 Cyclostomi and Dipnoi, may be taken as a starting-point. In 
 these, the notochord extends straight backwards to the hinder end 
 of the body and is surrounded quite symmetrically by the tail- 
 fin, which is therefore spoken of as protocercal or diphycerccd 
 (Fig. 30). This condition is also met with in many Fishes of the 
 
 FIG. 30. TAIL OF Protopterns. 
 
 Devonian strata as well as in young stages of Teleostei. In the 
 latter, however, the ventral half of the tail-fin with its sup- 
 porting skeleton (haemal arches and fin-rays) is, as a result of un- 
 equal growth, more strongly developed than the dorsal, and the end 
 of the vertebral column becomes bent upwards, thus giving rise to 
 a heterocercal tail. This form of tail may be recognised externally, 
 as in many Elasmobranchs, Ganoids, and numerous fossil Fishes ; or 
 may be masked by a more or less symmetrical tail-fin, as in Lepi- 
 dosteus (Fig. 31), Amia, and more particularly in most Teleosts L 
 (e.g. Salmo, Fig. 32), in which the heterocercal character is only 
 visible internally. The posterior end of the vertebral column 
 is then frequently represented by a rod-like urostyle, and in 
 Teleosts one or more wedge-shaped hypural bones (enlarged haemal 
 arches) generally occur directly beneath it (Fig. 32). 
 
 1 The term homocercal is sometimes used to describe the masked lieterocerca 
 condition of the tail in Teleostei. 
 
1 COMPARATIVE ANATOMY 
 
 Amphibia. The vertebral column of Urodeles may be differ- 
 entiated into cervical, tlwraco- lumbar, sacral, and caudal regions, 
 and these regions can be recognised, except in certain modified 
 forms, in all the higher Vertebrates. On account of the absence 
 -of extremities in Csecilians, the vertebral column can only be 
 
 FIG. 31. TAIL OF Lepidosteus. 
 
 divided into three regions cervical, thoracic, and a very short 
 caudal. In Anura, no special lumbar region can be recognised, 
 and the caudal portion is modified to form a urostyle (see pp. 41 and 
 44). The centra of the Amphibia, as well as those of the 
 Amniota, correspond to arch-centra (see p. 39). 
 
 FIG. 32. CAUDAL END OF VERTEBRAL COLUMN OF SALMON. (From Boas' s 
 
 Zoology. ) 
 
 h, centrum ; h', urostyle ; n, hasmal arch ; n' hypural bone ; o", neural arch ; t, 
 , neural spine. 
 
 The notochord of Urodele larvae, like that of most Fishes, 
 undergoes intravertebral constrictions, while intervertebrally it 
 grows thicker, and accordingly appears expanded. Thus the 
 vertebrae here also are amphiccelous. Later, intervertebral masses 
 of cartilage become developed, which, together with the bone 
 which is formed at the same time in the surrounding connective- 
 
VERTEBRAL COLUMN 
 
 tissue, extend inwards towards the centre, gradually constricting 
 the notochord so that it may eventually become entirely 
 obliterated. Finally a differentiation, as well as a resorption, 
 extending inwards from the periphery, occurs in these cartilaginous 
 parts : in the interior of each an articular cavity is formed, so that 
 in the vertebras of the higher Urodeles an anterior convexity and 
 
 ..u,t 
 
 1> 
 
 FIG. 33. LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN OF VARIOUS 
 URODELES. A, Ranodon sibericus ; B, Amblystoma tigrinum ; C, Gyrinophtiw 
 porphyriticufi (the three anterior vertebne, /, //, ///) ; D, Safamundrina 
 perspicillata. 
 
 'Oh, notochord ; Jrk, invertebral cartilage : OK, vertebral cartilage and fat-cells ; 
 K, peripheral bony covering of centrum ; R, ribs and transverse processes ; S, 
 vertebral constriction of notochord in Amblystoma tigrinum, without cartilage 
 and fat-cells in this region ; **, intervertebral cartilaginous tracts ; Mh, Mh, 
 narrow cavities ; Gp, Gk, articular socket and head ; Ligt, intervertebral 
 ligaments. 
 
 a posterior concavity may be distinguished, both covered with 
 cartilage ; they are, therefore, opisthoccdous. A glance at Fig. 33, 
 A to D, will make this clear. 
 
 In the development of the vertebral column of Urodeles we 
 can thus distinguish three stages: (1) A connection of the indi- 
 
44 
 
 COMPARATIVE ANATOMY 
 
 Oe 
 
 vidual vertebrae by means of the intervertebrally expanded 
 notoctiord ; (2) a connection by means of intervertebral masses- 
 of cartilage ; and finally (3) an articular connection. These 
 three different stages of development find a complete parallel 
 in the phylogeny of tailed Amphibians, inasmuch as many of 
 
 the Stegocephala of the Carboniferous 
 period, as well as the Perennibranchiata r 
 Derotremata, and many Salamanders, 
 possess simple biconcave vertebras without 
 differentiation of definite articulations. 1 
 
 The bony parts of the vertebrae of 
 Urodeles are not formed from the carti- 
 laginous sheath of the notochord, but in 
 the surrounding connective-tissue, there 
 being only an intervertebral cartilaginous 
 zone, extending into the ends of the centra. 
 In the Anura, on the other hand, as in 
 Elasmobranchs, Teleosts, bony Ganoids r 
 and the higher Vertebrata, the vertebrae 
 are preformed in cartilage, and true arti- 
 culations always arise between the 
 vertebrae : as a rule the convexity is 
 posterior and the concavity anterior (pro- 
 ccelous form). A further difference is 
 seen in the relations of the notochord, 
 which persists intravertebraliy longer 
 than intervertebrally, a condition which 
 leads towards the Reptiles. 
 
 The configuration of the caudal region, 
 of the vertebral column must also be re- 
 marked upon, as it differs in tailed and 
 tailless Amphibians. The long caudal 
 portion of the vertebral column in Frog 
 larvae, which is very similar to that of 
 Urodeles, undergoes during metamor- 
 phosis a gradual retrogressive change, 
 and the vertebrae of its proximal end 
 become fused together and ossified to form 
 a long unsegmented dagger -like bone, the 
 urostyle (Fig. 34). 
 
 Both neural and haemal arches arise 
 in direct continuity with the centra. 
 Haemal arches are, however, present in the caudal region of 
 Urodeles only. 
 
 The neural spines, as well as the transverse processes, which are 
 as a rule bifurcated at ttie base and are present from the second 
 
 1 In certain of the Stegocephala incomplete hoops of bone, the inter- and 
 pleuro-centra, twice as numerous as the arches, surrounded the persistent notochord.. 
 
 FIG. 34. VERTEBRAL 
 COLUMN OF Disco<jlo*u 
 pic t us. 
 
 Pa, articular processes ; 
 Pa, neural spine ; Pt, 
 transverse processes of 
 trunk vertebrae ; Ptc, 
 transverse processes of 
 caudal vertebras (uro- 
 style, Oc) ; 8 W, sacral 
 vertebra ; Ob, upper 
 arch of first vertebra ; 
 Sy, its condylar facets ; 
 Po, its anterior pro- 
 cess ; R, ribs. 
 
VERTEBRAL COLUMN 45 
 
 vertebra onwards, show the greatest variety as regards shape and 
 size, differing in the several regions of the body. The transverse 
 processes of the single sacral vertebra, which give attachment 
 to the pelvis, are particularly strongly developed, especially in the 
 Anura (Fig. 34). 
 
 Articular processes (zygapophyses, comp. p. 40) are well de- 
 veloped in all Vertebrates from TJrodeles onwards, and consist of 
 two pairs of projections arising from the anterior and posterior 
 edges respectively of the neural arch. Their surfaces are covered 
 with cartilage, and overlap one another from vertebra to vertebra 
 like tiles on a roof: not unfrequently, in Urodeles, the "neural 
 spines also articulate with one another, and thus a well- articulated 
 and mobile chain-like vertebral column results. 
 
 The first vertebra (and this is the only cervical vertebra of 
 Amphibia), becomes differentiated from the others, and consists of 
 a simple ring which articulates with the two condyles of the skull, 
 and also with the base of the latter by means of a more or less 
 marked process often spoken of as the " odontoid " process (Fig. 
 34) ; thus a freer movement between the skull and vertebral column 
 is rendered possible. This vertebra, however, is not homologous 
 with the first vertebra (i.e., the atlas) of the higher Vertebrates, 
 as is demonstrated by a study of its development, which shows 
 that the real atlas loses its individuality as a separate mass, and 
 becomes united with the occipital region of the skull. 1 The 
 first vertebra of Amphibians is therefore more nearly comparable 
 to one of the next following cervical vertebrae of higher forms. 
 It possesses posterior zygapophyses only, and its condylar facets 
 -correspond to modified transverse processes. 
 
 The number of vertebrae present in Urodeles is inconstant, and varies 
 greatly : it may reach to nearly 100 (Siren), and in Csecilians may be very 
 much greater (up to 275). In Anura, on the other hand, there are only 
 oight precaudal vertebras and one sacral, in addition to the urostyle. It is 
 evident from this fact alone that the recent forms of Urodela and Anura are 
 widely separated from one another. 
 
 Reptilia. In contrast to the numerous fossil forms, only a few 
 existing Reptiles, viz., Hatteria (Rhynchocephala) and the Geckos 
 (Ascalabota), retain throughout life the primitive biconcave char- 
 acter of their vertebrae, with the notochord expanded interverte- 
 brally. 
 
 In the generalised Rhynchocephala the formation of the vertebrae out of 
 several pieces, such as occurs amongst the Stegocephala (p. 44), is still indicated 
 by sutures, each vertebra consisting of two processes, a centrum proper 
 (pleurocentrum) and an iiitercentrum. 
 
 In all the others, the notochord remains expanded longer in the 
 intravertebral regions than intervertebrally, but in the adult it be- 
 
 1 A similar fusion of the anterior part of the vertebral column with the skull 
 occurs in some Fishes and in Dipnoi. ' 
 
46 COMPARATIVE ANATOMY 
 
 comes entirely aborted and replaced by bony tissue. This stronger 
 and more solid ossification of the whole skeleton forms a character- 
 istic difference between the Ichthyopsida on the one hand and 
 the Amniota on the other. As a rule the vertebrae of Reptiles 
 become definitely articulated with one another, and are of the 
 proccelous type : the above-named forms, with intervertebral re- 
 mains of the notochord, form an exception to this rule. In Croco- 
 diles fibro-cartilaginous intervertebral discs or menisci occur between 
 the centra (Fig. 35). 
 
 In Crocodiles the vertebra? are mostly procoelous, an exception being seen 
 in the two sacrals and first caudal. In Chelonians there is great variation in 
 the form of the individual centra of the cervical vertebrae even in the same 
 individual procoelous, opisthocoelous, biconcave, and even biconvex centra,, 
 with intervertebral discs, may occur ; while the thoracic and lumbar vertebrae 
 have flattened faces, and are firmly united together by cartilage. 
 
 In the Jurassic Ichthyosaurus and Eosaurus the centra were short and 
 deeply biconcave, like those of Fishes, and the arches were connected with 
 them by cartilage and connective tissue ; as a sacrum was absent, only a 
 precaudal and a caudal region can be recognised. In Plesiosaurus, Plio- 
 saurus, Nothosaurus, Simosaurus, the Anomodoiitia and others, the centra 
 were also biconcave or flattened. 
 
 What has been said as to the classification of the vertebrae into 
 different regions in Urodeles, as well as to the presence of the 
 various processes, usually applies here also to a still greater extent.. 
 Except in limbless form, there are always several cervical vertebrae 
 instead of a single one : there are also typically at least two sacral 
 vertebrae. The two first cervical vertebrae become differentiated 
 to form an atlas usually consisting of three pieces, and an axis- 
 with an odontoid bone (Fig. 35, and comp. p. 45). l 
 
 The neural spines vary in size, and transverse processes arise 
 from the centra themselves or close to them. Lower arches,, 
 attached intercent rally (chevron bones) are present in the tail in 
 Lizards, Crocodiles, and some Chelonians ; and besides these,, 
 median inferior processes of the centra themselves (? intercentra) are 
 seen in many of the vertebrae of Lizards, Crocodiles, and Snakes., 
 and in the latter paired processes partly enclose the caudal vessels. 
 The arches in Snakes, Lizards, and Chelonians become united 
 with the centra by synostosis, while in Crocodiles they remain, 
 separated from them by sutures (Fig. 35). 
 
 In consequence of the absence of a pectoral arch, the vertebral 
 column of Snakes and Amphisbsenians, like that of Csecilians,. 
 consists of trunk and caudal vertebrae only. The vertebral column 
 of Chelonians deserves particular notice as a large portion of it 
 becomes anchylosed with the dermal bones of the carapace (p. 33, 
 Fig. 21), and is thus rendered immovable. 
 
 1 The odontoid bone corresponds morphologically to a part of a centrum of the 
 atlas. A so-called pro-atlas the remains of a vertebra situated between the 
 skull and atlas proper is present in the Crocodilia (Fig. 35), Hatteria, and 
 Chamselaeo, as well as in many fossil forms. . 
 
VERTEBRAL COLUMN 47 
 
 In Snakes and some Lizards (Hatteria, Iguana) extra articular 
 processess (zygosphenes and zygantra) are developed on the neural 
 arches. In Hatteria and the Geckos, small separate ossifications 
 (intercentra, comp. p. 43, 44, 46) are present on the ventral side- 
 of the vertebra] column between many of the centra. In the caudal 
 region of Lizards anunossified septum remains in the middle of each 
 centrum, so that the tail easily breaks off at these points : when this 
 happens the tail grows again, but proper vertebrae are not formed. 
 
 In fossil Reptiles, which both as regards size and number of species 
 usually surpassed the existing representatives of the class, the sacrum often 
 consisted of as many as four or five vertebrae. The following facts will give 
 some idea of the monstrous proportions of these old genera of Reptiles : 
 Atlantosaunis immanis, a North American Dinosaur, reached a length of 
 
 Po 
 
 FIG. 35. ANTERIOR PORTION OF THE VERTEBRAL COLUMN OF A YOUNG 
 
 CROCODILE. 
 
 WK, centrum ; Ob, neural arch ; Ps, neural spine ; J-sr, intervertebral disc ; Pt r 
 transverse process, arising from the base of 'the arch and articulating with 
 the rib (jff, R l , R*) at f ; A, atlas ; u, ventral element, and 8 arch of atlas ; o, 
 "pro-atlas"; Ep, axis, articulating with the atlas at h; Po, odontoid 
 process. 
 
 about 80 feet, and the transverse diameter of the individual vertebra* 
 amounted to 16 inches, while Apatosaurus laticollis, found in the same strata, 
 possessed cervical vertebrae which reached a diameter of 3^ feet. 
 
 A knowledge of fossil genera of Reptiles is of the greatest interest, as in 
 many groups important points of connection with Birds can be recognised. 
 
 Birds. The vertebral column of Birds corresponds with that 
 of Reptiles not only in its phylogenetic relations, but also onto- 
 genetically. In both groups the notochord eventually disappears 
 entirely, and the whole skeleton becomes strongly ossified. 
 Archaeopteryx, as well as Ichthyornis (from the American Cre- 
 taceous), possessed biconcave vertebrae, but in existing Birds this 
 character never occurs except in the free caudal vertebrae (p. 49). 
 Cervical, thoracic, lumbar, sacral, and caudal regions can be distin- 
 guished. The arches always become united into a single mass 
 Avith the corresponding centra, and are no longer separated from 
 
48 COMPARATIVE ANATOMY 
 
 them throughout life by sutures, as is the case in certain Reptiles : 
 even the ligament which keeps the odontoid process of the axis 
 in its place may become ossified. Fibro-cartilaginous discs or 
 menisci are present between the centra. In the cervical region, 
 which is extremely flexible and often very long, the centra are 
 in nearly all cases connected by means of saddle-shaped synomal 
 articulations ; the upper part of the bifurcated transverse processes 
 arises from the arch, the lower from the centrum, and these may 
 unite with the corresponding forked rib, the vertebral artery and 
 vein extending through the foramen thus formed (Fig. 36). 
 
 In the thoracic and lumbar regions more or fewer of the 
 vertebras usually become immovably united together. 
 
 The sacral region in Bird-embryos, like that in existing adult 
 Reptiles, consists of two vertebras only, the transverse processes of 
 
 -Pa 
 
 FIG. 36, A. ATLAS AND Axis (from the left side) ; and B, THIRD CERVICAL 
 VERTEBRA (ANTERIOR FACE) OF WOODPECKER (Picus viridis). 
 
 A. Ob, A, arch and centrum of atlas ; f, condylar facet ; Po, odontoid process ; 
 
 WK, centrum of axis, and Sa, its saddle-shaped articular surface for the 
 third vertebra ; Ps, neural spine of axis ; Pt, transverse process. 
 
 B. Sa, articular surface of centrum ; Ob, upper arch ; Pa, articular process ; Pt, Pf, 
 
 the two bars of the transverse process, shown on one side anchylosed with 
 the cervical rib (R) ; Ft, vertebrarterial foramen ; Psi, median inferior pro- 
 cess (hyparpophysis). 
 
 which ossify separately and correspond to fused ribs, as in 
 Amphibians and Reptiles. During further development, however, 
 a number of other (secondary sacral) vertebras (thoracic, lumbar, 
 and caudal), with their rudimentary ribs, become fused with the 
 two primary ones (Fig. 37), so that the entire number of vertebras 
 in the sacrum may be as many as twenty-three. In Archasopteryx 
 the sacrum was much shorter than in existing Birds, and fewer 
 vertebras were united with it. 
 
 In existing Birds the caudal region always exhibits a more or 
 less rudimentary character, and in its posterior portion the ver- 
 tebras usually fuse together to form a flattened bone, the pygostyle, 
 which supports the tail quills (Fig. 111). An exception to this rule 
 is found only amongst the Ratitas, in which all the caudal vertebras 
 
VERTEBRAL COLUMN 
 
 remain distinct. That the latter is the more original condition in 
 Birds is shown by a study of their development as well as by the 
 condition of the tail in Archasopteryx, in which it was supported by 
 numerous elongated free vertebrae (Fig. 38,'. Moreover, in many 
 Birds (e.g. Psittacus undulatus) more vertebras are formed in the 
 embryo than are seen in the adult. It must, however, be borne 
 in mind that the pygostyle may be made up of from six to ten 
 fused caudal vertebras, and in the sacrum 
 even a greater number may be included 
 (cp. p. 48) : thus in the common Duck, 
 seven become united with the pelvis, 
 eight remain free, and the pygostyle is 
 composed of ten separately ossified and 
 fused segments, making in all twenty- 
 five vertebras originally present in the 
 caudal region of this Bird. 
 
 Mammalia. The notochord here 
 persists longer intervertcbrally than in- 
 travertebrally, but it disappears entirely 
 by the time the adult condition is reached. 
 A jelly-like pulpy mass, the nucleus 
 pulposus, persists, however, throughout 
 life in the centre of the fibre-cartilaginous 
 menisci which are developed between 
 the centra. The whole vertebral column 
 is preformed in cartilage, and the centra 
 develop in continuity with the arches 
 but become ossified from separate centres, 
 as do also the various processes. These 
 ossifications, however, become fused to- 
 gether in the adult. The presence of 
 bony discs or epiphyses on the ends of 
 the centra which unite with the latter 
 comparatively late, is very characteristic 
 of Mammals ; they are however absent 
 or only imperfectly developed in Mono- 
 trematas and in existing Sirenia. 
 
 True articulations between the centra 
 are never formed, except on the atlas 
 
 and anterior face of the axis; but as in Amphibians, Reptiles, 
 and Birds, well-developed articular processes (zygapophyses) 
 are present, arising from the neural arches. 1 The cervical 
 region is usually the most moveable, and the centra may be so 
 much hollowed out in this region as to give them an opisthoccelous 
 character (e.g. Ungulata,\ In some cases, on the other hand, the 
 
 1 In certain Edentata (e.g. Myrmecophaga, Dasypus) extra articular processes 
 are present besides the ordinary /ygapophyses on the posterior thoracic and 
 lumbar vertebra? (Fig. 39s. ). 
 
 E 
 
 FIG. 37. PELVIS OF OWL, 
 (Strix bubo). Ventral 
 view. 
 
 W 
 
 position of the primary 
 sacral vertebra : .be- 
 tween It and //, and 
 behind W, are seen the 
 secondary sacral verte- 
 brae, fused with the 
 primary ( W) ; II, ilium ; 
 Is, ischium ; P, pubis ; 
 t, foramen between 
 ilium and pubis ; If, 
 last two pairs of ribs. 
 
50 
 
 COMPARATIVE ANATOMY 
 
 cervical vertebrae may become firmly fused together (Cetacea). 
 The distal parts of the transverse processes, representing rudi- 
 mentary ribs, are perforated by the vertebrarterial canal (p. 48) : 
 in Monotremes these remain distinct at any rate for a long time. 
 
 FIG. 38. Archceopteryx lithographica. From the Solenhofen slates (Jurassic.) 
 After Dames, from the specimen in the Berlin Museum. 
 
 The atlas and axis essentially resemble those of Birds, but the 
 differentiation of the vertebral column into regions characterised 
 by difference of form is much more sharply marked than in any 
 other Vertebrates. There are as a rule seven cervical vertebra ; 
 Bradypus, however, possesses eight to nine, and Tamandua bivit- 
 tata eight, while in Manatus and Cholcepus there are only six. 
 
VERTEBRAL COLUMN 
 
 51 
 
 The transverse processes are simple in all but the cervical 
 region and arise from the base of the arch : in the thoracic region 
 they are tipped with cartilage on the ventral side of their distal 
 ends for articulation with the tubercle of the rib (p. 58). In the 
 lumbar and sacral regions 
 they arise from the centra, 
 and contain fused rib-ele- 
 ments. 
 
 In long-necked Ungulates 
 {e.g. Horse, Camel, Ox) the 
 neural spines of the anterior 
 thoracic vertebrae are greatly 
 developed, and a correspond- 
 ingly strong cervical ligament 
 (ligamentum nuchse) is par- 
 ticularly well developed to 
 support the weight of the 
 head. This is also true of 
 antler-bearing animals and of 
 the Gorilla. 
 
 The number of thoraco- 
 lumbar vertebrae varies greatly 
 in different Mammals : there 
 may be as few as fourteen 
 (Armadillo) or as many as 
 thirty (Hyrax). In Ungulates 
 the number is constantly 
 nineteen. In the lumbar ver- 
 tebrae the transverse pro- 
 cesses are especially long, 
 and other processes (anapo- 
 physes, metapophyses, hypa- 
 pophyses) are characteristi- 
 cally present in this region. 
 
 Thus, as in Amphib- 
 ians, Reptiles and Birds, 
 the pelvis is connected 
 with the sacrum by means 
 of rudimentary ribs. As 
 in the two last-mentioned 
 groups, there are not more 
 than two primary sacral 
 vertebras, but except in 
 Ornithorhynchus and most 
 Marsupials a few caudal 
 become later included in 
 the sacrum and are usually 
 more or less closely united 
 with it by synostosis. The various processes of the sacral vertebne 
 are more or less reduced. In Anthropoids, as in Man, the first 
 sacral vertebra is plainly marked off from the last lumbar by the 
 formation of the so-called promontory. A sacrum is wanting in 
 
 E 2 
 
 FIG. 39 A. DIAGRAM SHOWING MODE or OSSI- 
 FICATION OF HUMAN Axis. (Ventral 
 surface. ) From Flower's Osteology of the 
 Mammalia. 
 
 o, odontoid process, or centrum of atlas ; c, 
 proper centrum of axis ; na, neural arch ; 
 as, anterior articular surface ; e, e, e, e, epi- 
 physes, completing the ends of the centra. 
 
 tc 
 
 FIG. 39B. SIDE VIEW OF THE TWELFTH 
 AND THIRTEENTH THORAIC VERTEBRAE 
 OF GREAT ANTEATER (Myrmecophaga 
 jubata), f. From Flower's Osteology of 
 the Mammalia. 
 
 m, metapophysis ; tc, facet for articulation of 
 tubercle of rib; cc, ditto for -capitulum 
 of rib ; az, anterior zygapophysis ; az 1 , 
 additional anterior articular facet ; pz, 
 posterior zygapophysis ; pz l and pz-, addi- 
 tional posterior articular facets. 
 
52 COMPARATIVE ANATOMY 
 
 the Cetacea and Sirenia, in correspondence with the absence of 
 hind-limbs. 
 
 The caudal vertebrae vary extremely in their development, and 
 excepting in most long-tailed Mammals such as Kangaroos, 
 Sirenians, Cetaceans and certain Apes no longer develop lower 
 arches. When present these " chevron bones " are intervertebral 
 in position. 
 
 The greatest number of caudal vertebrae is found in Microgale longicauda 
 (forty-eight), Manis macrura (forty-six to forty-nine), Paradoxurus (about 
 thirty-six), and certain Monkeys (thirty- two to thirty-three). 
 
 The caudal region is most reduced in the higher Primates, in 
 which it gives rise to a stump-like coccyx consisting of at most five to 
 six rudimentary vertebrae, all fused together, and these may even (in 
 the human subject, especially in the male) fuse with the sacrum. 
 Many facts as regards the development as well as the structure of 
 the whole tail-region in the adult show however that the ancestors 
 of Man must have been provided with a distinct and functional 
 taiJ. 
 
 II. RIBS. 
 
 The ribs do not as a general rule (with the exceptions to be 
 noted presently) arise as outgrowths from the vertebral column, but 
 become developed independently in the skeletogenous layer that 
 is, in the tissue of the somites, and their connection with the 
 vertebral column is a secondary one. They stand in the closest 
 connection with the inter-muscular septa or myocommata of the 
 great lateral muscles of the body (Fig. 40 A,) are arranged seg- 
 mentally, and onto- as well as phylogenetically, pass through a 
 membranous, a cartilaginous, and a bony stage : their ossification is 
 independent of that of the vertebral column. In their primitive 
 form, the ribs have simple, unbifurcated heads, the articulation of 
 which with the vertebral column first takes place in the region of 
 the " intercentra " (p. 47), and from this condition all the later modi- 
 fications as regards their form and connection are to be derived. 
 
 The ribs present great .variation in the various vertebrate 
 Classes : they may be short and stump-like and almost horizontal 
 in position, or may grow ventral wards so as to encircle the body- 
 cavity. Primitively, ribs may be present all along the vertebral 
 column, but in the higher types they become reduced in certain 
 regions. 
 
 In order to arrive at sound conclusions as to the morphological 
 value of the ribs, their relations to the soft parts must be taken into 
 consideration. It is then seen that they are not completely homo- 
 logous throughout the vertebrate series, and that those of Ganoidei, 
 Teleostei and Dipnoi are not exactly comparable to those of Elasmo- 
 bianchii, Amphibia, and Amniota (Fig. 40A). 
 
/ 
 
54 COMPARATIVE ANATOMY 
 
 Ganoidei, Teleostei, and Dipnoi In these forms the ribs, 
 almost without exception, are connected with the ventral parts of 
 the notochordal sheath (Dipnoans) or with the " basal processes " 
 (Ganoids and Teleosts, see p. 38). l This is one point of difference 
 between the ribs of these forms and those of other Vertebrates : 
 another is that they are always situated beneath (internal to) 
 the lateral muscles, bet wee a these and the peritoneum (Fig. 40A, A, 
 B, C). In Teleosts the ribs are at first continuous with the basal 
 processes and become secondarily segmented off from them : this 
 may be a ccenogenetic modification. 
 
 Towards the caudal region, the ribs gradually take on the form 
 of haemal arches, which have precisely the relations of the ribs as 
 
 FIG. 40B. ANTERIOR END OF THE VERTEBRAL COLUMN OF POLYPTERUS. From 
 
 the ventral side. 
 
 WK, centra ; / V, first five pairs of dorsal ribs ; f t, ventral ribs. 
 
 described above. In Teleosts, however, the ribs gradually disappear 
 in passing backwards to the tail, and the ha3inal arches are formed 
 by the basal processes alone (Fig. 40A, B, c.). In spite of these differ- 
 ences, however, there can be no doubt that the ribs of Teleosts are 
 homologous with those of Dipnoans and Ganoids. 
 
 Large rib-like structures (" upper ribs") are present in Polypterus (Fig. 
 40s), which have a similar position to that of the ribs in the forms next to be 
 described ; and amongst the Teleostei (Clupeoidei, Salmonidte) small cartilages 
 are present beneath the lateral line in a similar position to that of the distal 
 
 1 The ribs are rudimentary in certain species of all the orders of Fishes, and 
 in some cases their place is taken by fibrous bands, arising from the skeletogenous 
 layer. They are wanting in Cyclostomes. 
 

 RIBS 55 
 
 ends of the upper ribs of Polypterus, to which they probably correspond. 
 There can be little doubt, however, that the more delicate bars which lie 
 veiitrally to the larger structures in Polypterus correspond to the ribs of 
 Fishes ("lower ribs ") as described above. It is therefore possible that 
 Polypterus and certain Teleostei possess the representatives of two sets of 
 ribs the one corresponding to those of the majority of Fishes, and the 
 other to those of Elasmobranchs, Amphibians, and Amniota (Fig. 40A, E). 
 
 The intermuscular bones present in the myocommata of Teleosts 
 probably correspond simply to ossifications of the septa, and have 
 nothing to do with ribs. 
 
 Elasmobranchii. The small, cartilaginous ribs of these 
 Fishes arise independently of the vertebral column in the connec- 
 tive tissue of the intermuscular septa, and extend outwards between 
 the dorso-lateral and the ventro-lateral muscles (see Fig. 40 A, 
 D). They are thus not genetically connected with the basal pro- 
 cesses, although they early become united to them by ligament, 
 and therefore do not correspond either to differentiations of 
 the hsemal arches or to transverse processes segmented off 
 from these. 
 
 Amphibia. The ribs of Amphibians arise in a very similar 
 manner to those of Elasmobranchs, but differ from them in being 
 from the first connected with the neural (dorsal) and not with the 
 hsemal (ventral) arches or basal processes. This is due to the 
 phylogenetic upward displacement of the longitudinal septum 
 separating the dorso-lateral from the ventro-lateral muscles. Like 
 those of Elasmobranchii and Amniota, the ribs are situated between 
 these two masses of muscle, but never extend very far laterally or 
 ventrally. 
 
 The ribs of Urodeles are forked at their proximal ends, and 
 articulate with bifurcated transverse processes of the vertebrae- 
 arising from the arch and centrum respectively : the dorsal part 
 of the transverse process, arising from the arch, is a new acquis- 
 ition. In many cases ribs are present only in the region of the 
 trunk, but occasionally they extend into the base of the tail, 
 where hsemal arches, corresponding to the basal processes of 
 Elasmobranchs, are also present 1 Urodeles therefore possess 
 the representatives of two kinds of ribs, morphologically distinct 
 from one another (comp. Polypterus and Teleostei, p. 54). All 
 the precaudal vertebra? except the first usually bear ribs ; in rare 
 cases (Spelerpes) there are a few ribless lumbar vertebras. 
 
 In the Anura the ribs are much shorter, and are doubtless 
 degenerated. As a rule, they become fused with the broad trans- 
 verse processes, at the ends of which they are situated ; the anterior 
 ones may sometimes, however, remain distinct (Fig. 34). They are 
 never bifurcated, and no trace of hsemal arches exists. 
 
 1 The elements of true ventral arches (basal processes) may also be present all 
 along the trunk in the larva of Salamandra maculosa, and are still more marked 
 in Necturus (Menobranchus). 
 
56 COMPARATIVE ANATOMY 
 
 In the Urodele Necturus four cartilaginous "abdominal ribs" (see 
 below) may be present in the septa between the ventral parts of the 
 myotomes on the level of the shoulder-girdle. Bony abdominal ribs also 
 occur in certain Stegocephala. 
 
 Reptiles. As already mentioned, the ribs of the Aniniota are 
 comparable to those of the Amphibia, but they grow further 
 ventral wards and so encircle the body-cavity to a greater or less 
 extent. The dorsal (proximal) section of the rib may also become 
 segmented from the distal (ventral) portion ; and as a rule a certain 
 number of the ribs unite together ventrally to form a sternum 
 (Fig. 44) : these are usually distinguished as " true " ribs from the 
 others, or " false " ribs. 
 
 The ribs of Snakes show the least amount of differentiation ; for, 
 without giving rise to a sternum, they extend along the whole 
 trunk from the third cervical vertebra to the anus, and retain 
 throughout a similar form and size. In Lizards, where a dorsal, 
 unforked, bony and a ventral cartilaginous portion can be distin- 
 guished, three or four ribs reach the sternum, and are not always 
 completely segmented off from it. 
 
 In Chelonians the cervical ribs unite with the, vertebras more or 
 less completely, and in the region of the trunk the ribs become fused 
 with the costal plates of the carapace (p. 33). Their proximal 
 unbifurcated ends are attached between the centra, at the junction 
 of centrum and arch. There is no sternum. 
 
 The proximal ends of the cervical ribs in the Crocodilia are 
 bifurcated, in correspondence with the double transverse processes 
 in this region, and thus a vertebrarterial canal is formed. Further 
 back, the ribs increase in length, and become segmented into two 
 or three articulated portions. In passing from before backwards, 
 their point of origin becomes gradually shifted, so that while the 
 anterior thoracic ribs are attached to the centra, the posterior ones 
 arise entirely from the transverse processes, which increase in size 
 correspondingly. Eight or nine ribs reach the sternum, and from 
 the eighteenth vertebra backwards the transverse processes no 
 longer bear ribs, but only short cartilaginous apophyses. 
 
 Uncinate processes (see below) are present in connection with 
 the ribs in the Crocodilia as well as in Hatteria. 
 
 "Abdominal ribs," arising as ossifications of the inscriptiones tendinea$ 
 of the ventral muscles, occur in Crocodiles and in Hatteria, as well as in 
 numerous fossil Reptiles. 
 
 Birds. The ribs of Birds exhibit a much more marked 
 segmentation into vertebral and sternal portions, both of which 
 become ossified, and this evidently stands in relation to their 
 more active respiration. Uncinate processes, moreover, arise from 
 the vertebral portions in nearly all Birds, and overlap the 
 ribs next behind them (Fig. 41). The whole costal apparatus 
 
RIBS 
 
 57 
 
 is rendered still firmer by the frequent fusion of the vertebrae 
 (p. 48), by the individual ribs often being very broad, as well as 
 by the form and arrangement of the sternum and pectoral arch, 
 which will be treated of later. The last three or four cervical 
 vertebrae may bear comparatively large and movable ribs. The 
 
 FIG. 41. SKELETON OF THE TRUNK OF A FALCON. 
 
 S, scapula ; G, glen old cavity for humerus ; Ca, coracoid, which articulates with 
 the sternum (St) at t ; Fu(Cl), furcula (clavicles) ; Cr, keel of sternum ; 
 V, vertebral, and Sp, sternal, portion of rib ; Un, uncinate process. 
 
 number of ribs which articulate with the sternum varies between 
 two (Dinornis elephantopus) and nine (Cygnus). 
 (Concerning the sacral ribs, see p. 48.) 
 
 Archseopteryx possessed 12-13 pairs of "abdominal ribs" (comp. p. 56). 
 
 Mammals. The cervical ribs here unite completely with the 
 vertebrae, and a vertebrarterial canal is thus formed, as in Croco- 
 diles and Birds. There is considerable variation with regard to the 
 
58 
 
 COMPARATIVE ANATOMY 
 
 number of ribs which reach the sternum, and in some cases the 
 sternal, as well as the vertebral ribs may become ossified. In both 
 " true " and "false" ribs (p. 56), a capitulum, a neck, a tubercul-um, and 
 
 a body may be distinguished (Fig. 42). 
 The capitulum of the former usually 
 articulates with its own centrum as well 
 as with that next in front, in the 
 region of the epiphysis ; the tuber- 
 culum articulates with the cartilagin- 
 ous facets on the transverse process. 
 In the " false " ribs, these characters 
 are to a greater or less extent lost. 
 As already mentioned (p. 51), rudi- 
 ments of ribs are present in the 
 lumbar and sacral regions, and unite 
 with the corresponding transverse 
 processes. 
 
 Fi. 42. COSTAL ARCH OF 
 MAN. 
 
 WK, centrum of vertebra ; 
 Pt, transverse process ; 
 Ps, neural spine ; Cp, 
 body of rib ; Ca, capitu- 
 lum ; Co, neck ; Tb, tuber - 
 culum ; Kn, cartilaginous 
 sternal rib ; St, sternum. 
 
 This fact, as well as the rudimentary 
 character and variety in size of the eleventh 
 and twelfth ribs and the occasional presence 
 of a thirteenth rib in Man, shows that a reduc- 
 tion in the number of these structures is here 
 taking place : a gradual shortening of the 
 thoracic portion of the vertebral column and 
 a corresponding lengthening of the cervical and lumbar regions is also 
 taking place in Mammals generally, and thus it may be stated that the 
 reduction in the number of ribs is correlated with a higher stage in 
 development of the Vertebrate body. 
 
 III. STERNUM. 
 
 Never present in Fishes, the sternum appears for the first time 
 in Amphibians in the form of a small variously -shaped plate of 
 cartilage situated in the middle line of the chest (Fig. 43). It 
 arises as a paired cartilaginous plate 1 in the inscriptiones tendineaB 
 of the rectus abdominis muscle, and therefore may be looked upon 
 as corresponding to a pair of "abdominal ribs." Such cartilaginous 
 abdominal ribs must have been present in greater numbers in 
 the ancestors of existing Urodeles (comp. Necturus, p. 56). In 
 many tailless Batrachians (e.g., Rana) the ventral portion of the 
 pectoral arch is continued forwards in the middle line, from where 
 the two clavicles meet, as a slender omosternum (Fig. 43, D) : 
 this has a similar origin, and the proximal portion both of it 
 and of the sternum become ossified. Thus the sternum and 
 omosternum of Amphibians are not to be considered as correspond - 
 
 1 It is unpaired from the first in Triton and Rana, but this is probably due to 
 an abbreviation of development. 
 
ill 
 
 FIG. 43. PECTORAL ARCH OF VARIOUS AMPHIBIANS. (From the ventral side). A Urodele 
 (diagrammatic) ; B Axolotl ; C Bombinator igneus ; D Rana esculenta, 
 
 SS, suprascapula ; S, scapula; Cl, procoracoid; Cl l (Cl in D], clavicle ; C, coracoid EC, 
 Co 1 , epicoracoid; t, Pf, G, glenoid cavity for the humerus ; St, St l , sternum; Ep, 
 omosternum ; Fe, fenestra between procoracoid and coracoid bars. * and t in B indicate 
 nerve-apertures. 
 
60 
 
 COMPARATIVE ANATOMY 
 
 ing to differentiations of the pectoral arch, 1 but as consisting of 
 skeletal parts which primarily belong to the body-wall, and only 
 secondarily come into connection with the limb-skeleton. 
 
 In most Urodeles and certain Anurans the edges of the cartilag- 
 inous sternum, are inserted into the grooved median margins of the 
 two coracoids (Fig. 43, B, C), to which they are united by connective 
 tissue. In Rana, on the other hand (D), in which the two halves 
 of the pectoral arch are much more closely connected in the 
 middle line, by far the greater part of the sternum lies entirely 
 posterior to the coracoids. In the Perennibranchiata and Dero- 
 tremata the sternum is much simpler than in other Amphibians, 
 and in Proteus and Amphiuma it undergoes complete degeneration. 
 
 FIG. 44. 
 
 -PECTORAL ARCH AND STERNUM OF A GECKO 
 (Hemidactylus verrucosus). 
 
 St, sternum ; /?, ribs ; Si, cartilaginous cornua to which the last pair of ribs is 
 attached ; SS, suprascapula ; 8, scapula ; Co, coracoid ; Co 1 , cartilaginous 
 epicoracoid : Ep, episternum ; a, ft, c, membranous fenestra? in the coracoid ; 
 Cl, clavicle ; G, glenoid cavity for the humerus. 
 
 Nothing is known with regard to the sternum of fossil Amphibians, 
 which was probably entirely cartilaginous. 
 
 In the Amniota, the sternum arises by a number of ribs on 
 either side of the middle line running together to form a continuous 
 cartilaginous tract. An unpaired cartilaginous sternal plate is 
 formed by the tract of either side becoming more or less completely 
 fused with its fellow, and from this plate the ribs become 
 secondarily segmented off by the formation of true articulations. 
 
 1 It has been recently shown that in the Elasmobranch Notidanus cartilages 
 are present in the median ventral line of the pectoral arch which are segmented 
 off from the coracoids. 
 
STERNUM 
 
 61 
 
 Later it may become calcified (Reptiles), or converted into true 
 bone (Birds, Mammals). In Reptiles, Birds, and Monotremes the 
 coracoids, as in Amphibians, always come into direct connection 
 with the lateral edges of the sternum (comp. Figs. 41, 44, and 48). 
 The sternum is greatly developed in Birds, and consists of a 
 broad more or less fenestrated plate, provided in the vast majority 
 of Carinatse with a projecting keel, which forms an additional 
 surface for the origin of the wing-muscles (Fig. 41). In contrast 
 to these, the cursorial RatitaB are characterised by a broad, more 
 or less arched, shield-like sternum without a keel. In some 
 flightless CarinataB, however, the keel is rudimentary or even ab- 
 sent, and a keel may occasionally appear, though not constantly, 
 
 A B C 
 
 FIG. 45. A, STERNUM OF Fox ; B, OF WALRUS; AND C, OF MAN. 
 
 Mb, manubrium ; C, body ; Pe, xiphoid process ; If, ribs. 
 
 in certain Ratitae. The presence or absence of a keel is not, there- 
 fore, a constant character separating these two groups of Birds 
 from one another. 1 
 
 A greater number of ribs are as a rule concerned in the forma- 
 tion of the sternum of Mammals than is the case in Reptiles and 
 Birds. Consisting at first of a simple cartilaginous plate, the 
 sternum later becomes segmented into definite bony portions 
 (sternebras) the number of which may correspond to the affixed 
 ribs (Fig. 45, A, B). But in other cases, as, for instance, amongst 
 Primates (C), the individual bony segments may run together to 
 form a long plate (corpus sterni). The anterior end of the sternum 
 becomes differentiated into the so-called mamibrmm, and the 
 posterior end into the xiphoid or ensiform process. The latter owes 
 
 its origin in the embryo to the ventral fusion of a true pair of ribs. 
 
 A 
 
 1 A keel was also present in the flying Reptile Plesiosaurus, and may be 
 developed wherever a larger surface for the origin of the pectoral muscles is 
 required (e.g., Cheiroptera). 
 
62 
 
 COMPARATIVE ANATOMY 
 
 IV. EPISTERNUM. 
 
 Episternal structures, which are wanting in Fishes, Dipnoans 
 and recent Urodeles, play an important part in fossil Amphibians 
 
 FIG. 46. PECTORAL ARCH or VARIOUS STEGOCEPHALA (from the ventral side). 
 
 After H. Credner. 
 
 A, Branchiosaurus, x 3 ; B, Pelosaurus x 2 ; C, Discosaurus, x 2 ; D, Hylono- 
 mus, x 2 ; E, Archegosaurus, x about \. Ej)S, episternum ; Cl, clavicle ; 
 #, scapula ; c, coracoid ; s, calcification in the sternum or in the cartilage 
 of the coracoid. 
 
 and primitive Reptiles (e.g., Stegocephala and Palaaohatteria), in 
 which, both as regards form and structure, they bear a great re- 
 semblance to the episternum of certain existing Reptiles. 
 
EPISTERNUM 
 
 63 
 
 FIG. 47. PECTORAL ARCH OF PAL^OHAT- 
 TERIA (from the ventral side) After 
 Credner. 
 
 In the Stegocephala , the episternum (" interclavicle ") consists 
 of a large bony plate, situated ventrally to the sternum, some of 
 the various forms of which, 
 as well as its relation to 
 the pectoral arch and more 
 particularly to the clavi- 
 cles, will be seen by refer- 
 ence to Fig. 46. 
 
 The episternum of 
 Palceohatteria and of re- 
 cent Lizards and Crocodiles 
 is essentially similar to 
 that of the Stegocephala 
 (Figs. 44, 46, and 47). In 
 Lacerta and Crocodilus it 
 arises, from before back- 
 wards, as a paired structure, 
 which is not preformed in 
 cartilage. An episternum 
 
 is wantino" in Chelonia and S, scapula ; C, coracoid ; C7, clavicle ; Eps, 
 Ophidia,as well as in episternun. 
 
 Chamseleo and Anguis. 
 
 In Birds no independent elements corresponding to this 
 structure can be recognised ; the ligament extending between 
 the clavicles and the sternal keel, the periosteal covering of the 
 
 keel which is continued backward 
 from this ligament, and the median 
 portion of the fused clavicles when 
 separately ossified (" interclavicle ") 
 may possibly have something to do 
 with an episternum without being 
 exactly homologous with it. 
 
 The origin and meaning of the 
 mammalian episternum, which is pre- 
 formed in cartilage, is not known ; it 
 has probably no direct connection 
 
 ** 9 \Cr~~- r - c - w ^ ^ e s i m ^ ar ly~ name d structure in 
 
 Reptiles, but apparently agrees with 
 the latter at any rate as regards 
 position and relations in the embryo 
 Mole (Fig. 47A). 
 
 In Monotremes (Fig. 48) and certain 
 Marsupials a median and two lateral 
 portions can- be distinguished, the 
 latter being in connection with the 
 clavicles. In these Marsupials the 
 median portion unites with the ster- 
 num, and as in Monotremes, becomes 
 
 d. 
 
 FIG. 47 A. EPISTERNUM OF AN 
 EMBRYO MOLE. (After 
 A. Gotte). 
 
 St, sternum ; es 1 , central por- 
 tion and es", lateral por- 
 tion of the episternum ; 
 cl, clavicle ; r.c, ribs. (The 
 figure was constructed 
 from two consecutive hori- 
 zontal sections.) 
 
64 COMPARATIVE ANATOMY 
 
 ossified; while the lateral portions remain cartilaginous. In 
 other Marsupials various stages in the reduction of the episteraum 
 are met with. 
 
 Amongst the Placentalia the episternum is retained in the most 
 independent condition in certain South American Cavies as well as 
 in the Porcupine and other Rodents in which it consists of a median 
 and two lateral parts, which are, however, quite independent of one 
 another, and are only connected by ligaments. The median, 
 
 FIG. 48. PECTORAL ARCH or Ornithorhynchus paradoxus. 
 
 m.s, manubrium sterni ; c 1 , c 2 , c 3 , first, second, and third ribs ; at, sternebra ; sc, 
 scapula ; ra.c, coracoid (metacoracoid) ; e.c, epicoracoid ; d, clavicle; es 1 nd 
 es 2 , episternum (" interclavicle"). 
 
 cartilaginous portion is closely applied to the sternum, while the 
 lateral portions are connected with the clavicles, 
 
 In the Sciuromorphse and Myomorphse the episternal apparatus is still 
 further modified, the median piece having disappeared (or more probably 
 having united with the sternum), while the small lateral pieces are attached 
 to the manubrium and in the Sciuromorphre articulate with the clavicles. 
 In the Lagomorphse fibre-cartilaginous lateral portions only are present, 
 extending as far as the clavicles. 
 
 V. THE SKULL. 
 
 Introduction. 
 
 The question as to the primary origin of the skull in the 
 Craniata has always taken a foremost place amongst the morpho- 
 logical problems relating to the structure of Vertebrates. Until 
 past the middle of the present century the theory which held the 
 field was the "vertebral theory" of Goethe and Oken, according to 
 
THE SKULL 65 
 
 which the skull consisted of a number of modified vertebrae 
 (" cranial vertebrae "). On this theory, therefore, the skull Avas 
 regarded as a special modification of the anterior part of the 
 vertebral column, and a large number of facts were brought 
 forward in support of it : even when morphological science had 
 made further considerable advances, there still seemed to be a 
 certain amount of justification for it. 
 
 The arguments in support of the vertebral theory of the skull 
 may be briefly stated as follows. As in the vertebral column, three 
 stages may be distinguished in the skull, ontogenetically as well as 
 phylogenetically: viz., a membranous, a cartilaginous, and a bony stage 
 (comp. p. 36). There is thus an important correspondence between 
 these two parts of the cranio- spinal axis, and this is further em- 
 phasized by the fact that the notochord always extends for a certain 
 distance into the base of the skull, so that the latter is developed on 
 the same skeletogenous basis as. and in direct continuation of, 
 the vertebral axis. 
 
 This theory depended on giving an exact account merely of the 
 skeletogenous elements taking part in the formation of the skull, 
 and for a long time it was not recognised that this could not 
 possibly lead to a true interpretation of the origin of the verte- 
 brate head. To attempt to do so was to " put the cart before the 
 horse," by looking upon the last acquisition of the head its skeleton 
 as the leading point for future researches. 
 
 It was only very gradually ascertained that the skull has never 
 consisted of segmentally arranged cartilaginous portions, either in 
 the course of its ancestral history or in that of the development of 
 the individual. In the occipital region alone did it possibly at one 
 time possess distinct neural arches, owing to the assimilation of 
 more or fewer of the anterior segments of the tiunk ; and the 
 view gradually gained ground that this important problem could 
 not be solved merely by an anatomical and embryological analysis 
 of the skeleton, but that a number of other parts and organs 
 which arise much earlier must also be taken into account 
 and their origin traced : such are, the sensory organs, brain and 
 cerebral nerves, cranial muscles, and the anterior part of the 
 alimentary canal together with the mouth and visceral clefts. 
 
 A. considerable advance was thus made, and the problem was 
 vigorously attacked both from the anatomical and embryological 
 sides ; and many of the researches which resulted have become 
 classical in the history of the subject. It is impossible here to 
 give more than the barest outlines of the results obtained, and 
 even now much remains to be elucidated in this complex question, 
 about many details of which numerous differences of opinion still 
 exist. Moreover, a knowledge of the development and distribu- 
 tion of the cerebral nerves is a necessary preliminary to the study 
 of cranial morphology : these are treated of in a subsequent 
 chapter, to which the reader is referred for explanation of parts of 
 the following paragraphs. 
 
66 COMPARATIVE ANATOMY 
 
 The portion of the skull which is situated along the main 
 axis in continuation of the vertebral column and which encloses 
 the brain is known as the brain-box or cranium, and is primarily 
 composed of cartilage. A series of cartilaginous arches arise in 
 serial order on the ventral side of the brain-case ; these encircle 
 the anterior part of the alimentary tract like hoops, incomplete 
 dorsally, and are distinguished from the cranial region as the 
 visceral skeleton. The latter stands in important relation to 
 branchial respiration, inasmuch as each consecutive pair of arches 
 encloses a passage (gill-slit), communicating between the pharynx 
 and the exterior ; this is lined by endoderm, and through it the 
 water passes in branchiate forms. The foremost visceral arch 
 bounds the aperture of the mouth, thus forming a firm support 
 for it, and giving rise to the skeleton of the jaws; the other arches 
 function primarily as gill-supports. Both cranial and visceral 
 portions may become ossified later. 
 
 Before the cartilaginous skeleton begins to be formed in the 
 embryo, the greater part of the head consists of a soft, mesoblastic 
 formative tissue, which gives rise to a membranous capsule around 
 the brain : the individual cerebral nerves can already be plainly 
 distinguished (membranous stage, comp. p. 36). The three organs 
 of the higher senses also appear at a very early stage ; and these, in 
 the course of further development, come to be situated in definite 
 bays or cavities within the head, and thus are of extreme im- 
 portance in modifying the configuration of the skeletal structures 
 which are formed around them later. 
 
 In the embryos of lower Vertebrates (e.g., Elasmobranchs) 
 more or less of the mesoblastic tissue which surrounds, isolates, 
 and supports these organs becomes divided up metamerically into 
 segments, so that a segmentation into somites (protovertebrce) occurs 
 in the posterior part of the head as well as in the body (comp. 
 pp. 8 and 36). The mesoblastic segments of the head, some of 
 which enclose cavities arising from the ccelome (or the pre-oral 
 gut), consist of a tissue from which later become differentiated 
 all the supporting structures including, of course, the skull, as 
 well as the muscles (myotomes}. Without going into further 
 details as to the number and fate of these segments and their 
 relation to the cerebral nerves, concerning which there is con- 
 siderable diversity of opinion, it may be stated that the primary 
 segmentation of the part of the head posterior to the auditory 
 organ, in the region of the vagus and hypoglossal nerves, is at any 
 rate more pronounced than that of the more anterior part of the 
 head. 
 
 The relations of the visceral to the cranial skeleton, and those 
 of both to the primary segmentation of the head, must also be 
 taken into consideration. Both cranial and visceral regions must 
 have been originally segmented, and each myotome at one time 
 included a ventral portion (lateral plate of the mesoblast) 
 which enclosed a corresponding section of the cranial ccelome, or 
 
THE SKULL 
 
 67 
 
 "head-cavity." Later, however, the visceral region became re- 
 latively shifted to a greater or less degree, especially in the anterior 
 part of the head, so that its segments no longer corresponded to- 
 those of the cranial region. Thus we find that the segmentation of 
 the nervous, muscular, and visceral parts of the head do not correspond 
 with one another. But although the segmentation of the visceral 
 portion of the skull has in the course of phylogeny reached a certain 
 degree of independence, and the cranial portion alone can be looked 
 upon as being made up of a series of somites, it must not be 
 forgotten that mesoblastic tissue extends from the head-somites 
 into the visceral arches, each of the two anterior of which still 
 contain a ccelomic cavity at a certain period of development. 
 
 a. Brain-Case (Cranium). 
 
 The first cartilaginous rudiments appear in the primitively 
 membranous skull-tube in the form of a pair of rods, the trabeculw 
 cranii. These lie along the base of the brain, their posterior 
 parts embracing the notochord ; they are thus divisible into pro- 
 chordal (anterior) and parachordal 
 (posterior) regions (Fig. 49), which 
 may be continuous with one another. 
 The parachordals soon unite to form a 
 lasilar plate, which grows round the 
 notochord dorsally and ventrally, and 
 thus early forms a solid support for 
 the hinder part of the brain. The 
 slender trabeculse project forwards and 
 enclose a space, which may be spoken 
 of as the primitive pituitary space 
 (Fig. 49). 
 
 These structures may undergo 
 further development in many dif- 
 ferent ways in the various Vertebrate 
 groups : the trabeculse may become 
 completely united with one another 
 in the median line (Fig. 50, A), and 
 the connective-tissue of the oral 
 mucous membrane may become ossi- 
 fied to form a parasphenoid (B). In 
 other cases, the trabeculse may become 
 compressed and partly aborted owing 
 to the great development of the eyes : 
 this obtains, e.g., in certain Reptiles 
 and in all Birds, in which a fibro-cartilaginous interorbital septum 
 appears in their place (C). 
 
 In most cases a median cartilaginous bar (intertrdbeetda) is 
 formed between the trabeculse in front, fusing with them, and 
 
 F 2 
 
 FIG. 49. FIRST CARTILAGINOUS 
 RUDIMENTS OP THE SKULL. 
 
 C, notochord ; PE, separate 
 parachordal elements ; Tr, 
 trabeculse cranii ; PR, pitui- 
 tary space; ^V, A, 0, the 
 three sense-capsules (olfac- 
 tory, optic, and auditory). 
 
68 
 
 COMPARATIVE ANATOMY 
 
 forming the ethmo-nasal septum (Fig. 51). It occasionally projects 
 forwards to form a rostrum (Figs. 55, 56, and 58). 
 
 FIG. 50. DIAGRAMMATIC TRANSVERSE SECTIONS OF THE HEAD IN EMBRYO 
 (A) STURGEONS, ELASMOBRANCHS, ANURANS, AND MAMMALS ; (B) URODELES 
 AND SNAKES ; (C) CERTAIN TELEOSTEANS, LIZARDS, CROCODILES, CHELONIANS, 
 AND BIRDS. 
 
 Tr, trabeculse cranii ; G, brain ; A , eyes ; Ps, parasphenoid ; IS, interobital 
 septum ; F, frontal ; Off, olfactory nerve. 
 
 We must now further follow the processes of growth, start- 
 ing from the primary condition described above, in which 
 
 the trabeculse have united together in 
 the middle line. The cartilaginous 
 basal plate now comes into relations 
 with the olfactory, optic, and auditory 
 organs (Fig. 51), around which carti- 
 laginous capsules are formed. Thus an 
 olfactory, an orbital, and an auditory 
 region are early differentiated. 
 
 The olfactory and auditory capsules, 
 especially in higher types, then become 
 more and more drawn in to the skull 
 proper, and the lateral edges of the 
 basal plate begin to grow upwards 
 round the brain on both sides, eventu- 
 ally extending even to the dorsal region. 
 Thus a continuous cartilaginous capsule 
 is formed, such as persists throughout 
 life in Elasmobranchs for example. 
 But in by far the greater number of 
 Vertebrates, the cartilage does not play 
 so great a part, and is, as a rule, con- 
 fined to the base and lower parts of 
 the sides of the skull and to the sense- 
 capsules, except in the occipital region, 
 where it always extends over the brain. 
 The rest of the skull, more particularly 
 the roof, becomes directly converted 
 from membrane into bone. At the 
 same time, bones may become differ- 
 entiated in connection with the primary 
 
 FIG. 51. LATER STAGE IN THE 
 DEVELOPMENT OF THE PRIM- 
 ORDIAL SKULL. 
 
 C, notochord ; B, basilar plate ; 
 Tr, trabecula, which has 
 united with its fellow in 
 front of the pituitary space 
 to form the ethmo-nasal sep- 
 tum (S) ; Ct, cornu trabe- 
 culse, and AF, antorbital 
 process, which support the 
 olfactory organ (NK) in 
 front and behind ; Ol, for- 
 amina for exit of the olfac- 
 tory nerves from the crani- 
 um ; PF, postorbital pro- 
 cess of trabecula ; A , eye ; 
 O, auditory organ. 
 
THE SKULL 
 
 69 
 
 cartilaginous skull (chondrocraniuiii) itself, which is thus more or 
 less completely replaced by an osteocranium. In general the 
 higher the systematic position of the animal, the less extensive 
 are the cartilaginous constituents and the more important the 
 bony. 
 
 &. The Visceral Skeleton. 
 
 The primarily cartilaginous visceral arches encircle the anterior 
 section of the alimentary canal, lying embedded in the inner part 
 of the walls of the throat (Figs. 52 and 53) and usually becoming 
 ossified latter. They are always present in a greater number (up 
 to as many as nine) in forms which 
 possess gills than in higher types 
 (Amniota), in which they gradually be- 
 come reduced, and may undergo a 
 change of function, certain of them in 
 some cases taking on definite relations 
 to the auditory organ and larynx. 
 
 The most anterior arch, serving as 
 a support for the walls of the mouth 
 and receiving its nerve supply from the 
 trigeminal, arises first, and is distin- 
 guished from the other or post-oral 
 arches as the mandibular arch. The 
 post-oral arches only function as gill- 
 bearers in the Anamnia : even the first 
 of these, the hyoid, which is supplied by 
 the facial nerve, becomes modified from 
 those lying behind it: the latter, or 
 branchial arches proper, are supplied by 
 the glossopharyngeal and vagus. All 
 the visceral arches must originally, how- 
 ever, have borne gills. 
 
 FIG. 52. DIAGRAMMATIC 
 TRANSVERSE SECTION OF A 
 STILL LATER STAGE IN THE 
 DEVELOPMENT OF THE 
 PRIMORDIAL SKULL. 
 
 Primarily unsegmented, the indi- 
 
 notochord ; Tr, trabeculae, 
 which enclose the brain 
 (C) ventrally and later- 
 ally; 0, auditory capsule ; 
 RH, the cavity of the 
 pharynx, enclosed by the 
 visceral skeleton ; 1 to 4, 
 the individual elements 
 composing each visceral 
 arch, which is united 
 with its fellow by a basal 
 piece (Cp). 
 
 vidual post-oral arches may become 
 
 broken up into as many as four pieces, 
 
 of which the uppermost becomes inserted 
 
 under the base of the skull, while the 
 
 lowermost is connected with its fellow by a median basal piece 
 
 (Fig. 52). 
 
 The mandibular arch also undergoes segmentation, and becomes 
 divided into a short proximal piece, the quadrate, and a long distal 
 mandibular or Meckel's cartilage (Fig. 53). The quadrate grows 
 out anteriorly into a process, the palatoqiiadrate or palatopterygoid, 
 which usually becomes fixed to the base of the skull and forms 
 the primary upper jaw, Meckel's cartilage forming the lower jaw. 
 
70 
 
 COMPARATIVE ANATOMY 
 
 The quadrate, which serves as a support (suspcnsoriuni) for the 
 jaws, either remains separated from the skull by an articulation 
 that is, is only united to it by connective-tissue or it forms one 
 mass with it. 
 
 The hyoid which has always close relations with the man- 
 dibular arch, and may also take part in its suspensorial apparatus x 
 
 br 
 
 warn 
 
 / 
 
 r c 
 
 i b c cL e 
 
 FIG. 53. DIAGRAMMATIC FIGURE OF AN EMBRYONIC ELASMOBRANCH SKULL, 
 SHOWING THE RELATIONS OF THE VISCERAL ARCHES. 
 
 JV, nasal capsule ; A , eye ; 0, auditory capsule ; Tr, trabecula ; Q and PQ, 
 quadrate and palatopterygoid, which are bound to the trabecula by ligameiits 
 at t ; M , Meckel's cartilage ; L, labial cartilages ; H, hyomanelibular ; K, 
 hyoid arch ; a to e, branchial arches, between which the gill-clefts (/to V) are 
 seen ; S, spiracle ; C, notochord ; b, vertebrae, br, brain ; sp.c, spinal cord. 
 
 becomes divided, as do the true branchial arches, into a number 
 of segments, the upper of which in many Fishes is distinguished as 
 the hyomandibidar (Fig. 53), from which a sympicctic may be 
 differentiated distally. In the mid- ventral line there is a basi-hyal 
 connecting the arch of either side, and embedded in the tongue 
 (entoglossal or glossoliyal). 
 
 c. The Bones of the Skull. 
 
 It is usual and convenient to distinguish in the entire skeleton 
 between the bones which are formed in connection with cartilage, 
 and eventually replace it to a greater or less extent (cartilage 
 
 1 It appears to be probable that the hyomandibular and hyoid proper are 
 separate in origin : possibly also the spiracular cartilage (p. 75), often looked 
 upon as representing fused mandibular rays, represents the remains of an entire 
 arch ; and Dohrn maintains that Meckel's cartilage and the palatoquadrate each 
 represents a distinct arch. 
 
THE SKULL 71 
 
 bones), and those which arise in connective-tissue, entirely inde- 
 pendent of cartilage (membrane- or investing -bones). But it must 
 be borne in mind that there is no hard and fast line between 
 these, and that histologically they are indistinguishable from one 
 another. Bone is always phylogenetically formed outside the 
 cartilage, and its first appearance within cartilage (as in the 
 Amniota more particularly) is to be looked upon as a secondary 
 condition. 1 Again, in other cases (e.g., in parts of the skeleton of 
 Elasmobranchs), true bones are not formed at all, there being 
 only a calcareous incrustation of the cartilage (calcified cartilage). 
 
 The bones arising in the membranous regions of the skull 
 (including the perichondrium) primarily come under the category 
 of the dermal skeleton and, as already mentioned with regard to 
 the latter, are to be looked upon as originating phylogenetically 
 in connection with dermal denticles (p. 30). In this manner 
 the bones of the mouth-cavity of Fishes and Amphibians, for 
 instance, still arise : it must be remembered that the epithelium 
 of the oral cavity is formed by invagination of the outer skin. 
 
 Such a mode of origin of the first skull-bones appears to be the 
 oldest and most primitive, and it is most apparent in the lower 
 Vertebrates (Fishes). This holds good also for those cases in which 
 bones are formed merely by deposition of calcareous matter directly 
 in the connective-tissue layer, without giving rise to tooth-struc- 
 tures (e.g., all investing bones) probably owing to an abbrevia- 
 tion of development. 
 
 The following lists give a summary of the most important 
 bones according to their different relations to the skull. 
 
 I. Investing Bones of the Mouth-Cavity (partly lying 
 within it, partly bounding it on the outer side). 
 
 1. Para sphenoid. 
 
 2. Vomer. 
 
 3. PremaxilJa. 
 
 4. Maxilla. 
 
 5. Jugal. 
 
 6. Quadratojugal (in part). 
 
 7. Dentary. 
 
 1 The different varieties of ossification may be conveniently classified as 
 follows : 
 
 I. " Membrane Bones/' (a) Dermostoses ossifications of thedermis; (ft) 
 parostoxes ossifications of the looser subcutaneous tissue ; (c) ectostoses ossifi- 
 cations of the inner layer of the fibrous investment (perichondrium) of a tract of 
 cartilage : these may extend into the latter, replacing it, and thus give rise 
 secondarily to 
 
 II. " Cartilage Bones," (endostose*). 
 
 It may, however, happen that in the course of generations an investing bone 
 may take the place of a cartilage bone, and the formation of cartilage be entirely 
 suppressed and not repeated again oiitogenetically. 
 
72 COMPARATIVE ANATOMY 
 
 8. Splenial. 
 
 9. Angular. 
 
 10. Supra -angular. 
 
 11. Coronoid. 
 
 12. Palatine. 
 
 13. Pterygoid. 
 
 II. Investing Bones of the Outer Surface (enumerated 
 from before backwards). 
 
 1. Nasal. 
 
 2. Lachrymal. 
 
 3. Frontal. 
 
 4. Prefrontal (of Reptiles). 
 
 5. Postfrontal or post orbital. 
 
 6. Supraorbital. 
 
 7. Parietal. 
 
 8. Temporal or squamosal. 
 
 9. Supraoccipital (in part). 
 
 III. " Cartilage Bones." 
 
 2. IShenS i Pre f n , fc OJ ?* Amniota (forming the base 
 
 3. Presphenoid J of the skull). 
 
 4. Exoccipital (and supraoccipital, in part). 
 
 5. Pro-, epi-, and opisthotic, also (in Teleostei) sphenotic and 
 
 pterotic (forming the bony auditory capsule). 
 
 n ' *i- j- sphenoid, developed in the trabecular region. 
 
 8. Ethmoid, together with the rest of the skeleton of the nose 
 
 (turbinals, &c.). 
 
 9. Quadrate. 
 
 10. Articular. 
 
 11. Visceral skeleton (in part). 
 
 ANATOMY OF THE SKULL. 
 
 SPECIAL PAET. 
 
 A. Fishes. 1 
 
 In the Cyclostomata. the skull is developed essentially in the 
 manner already described. Later, however, it shows many special 
 peculiarities, probably in consequence of the suctorial (Petromyzon) 
 
 1 In Amphioxus (Acrania) there is no cranial skeleton, and the elastic non- 
 cartilaginous rods which support the branchial apparatus are not comparable with 
 the visceral skeleton of the Craniata. 
 
THE SKULL 
 
 73 
 
 or parasitic (Myxine) mode of life of these animals : the most 
 important of these is the absence of jaws such as are present in all 
 other Craniata; for this reason these forms are spoken of as 
 Cyclostomata to distinguish them from the other craniate Verte- 
 brates or Gnathostomata. Instead of the jaw-apparatus, which 
 has doubtless become degenerated, and indications of which as well 
 as of the hyoid can apparently still be seen (Fig. 54, sb.oc.a, 
 p. lat.c, sty-.c, cn.c), a number of cartilages are present supporting 
 the anterior part of the head. In the adult Lamprey, for instance, 
 the suctorial mouth is supported by various skeletal elements, 
 amongst which may be mentioned a ring-like cartilage around the 
 margin of the dome-shaped oral funnel, between the dorsal side 
 
 L.C.I 
 
 til at. 
 
 br.cl.i 
 
 FIG. 54. SKULL WITH BRANCHIAL BASKET OF Petromyzon marinns. 
 (After W. K. Parker.) 
 
 The cartilaginous parts are dotted. a.d.c. anterior dorsal cartilage; a.lat.c. 
 anterior lateral cartilage; an.c. annular cartilage; au.c. auditory capsule; 
 br.b. 1 7, vertical bars of branchial basket; br.cl. 1 7, external branchial 
 clefts; cn.c. cornual cartilage; cr.r. cranial roof; I.e. 1 <, longitudinal 
 bars of branchial basket; Ig.c. lingual cartilage; m.v.c. median ventral 
 cartilage; na.ap. nasal aperture; nch. notochord; JW. 2, foramen for optic 
 nerve ; olf. c. olfactory capsule ; pc. c. pericardial cartilage ; p. d. c. posterior 
 dorsal cartilage; p.lat.c. posterior lateral cartilage; sb.oc.a. sub-ocular 
 arch ; st.p. styloid process ; sty.c. styliform cartilage ; t. teeth. 
 
 of which and the brain-case are a couple of large overlapping 
 cartilages : the tongue is supported by a long, lingual cartilage. 
 On the mucous membrane covering the annular and lingual 
 cartilages inside the oral funnel are a number of horny teeth. 
 The fibre-cartilaginous olfactory sac is unpaired, and opens 
 on the dorsal surface of the head by a single nostril. The 
 visceral skeleton also shows many exceptional peculiarities : it 
 consists of a delicate cartilaginous basket-work (Fig. 54), and has 
 a very superficial position ; we may * accordingly speak of the 
 unsegmented cartilages of which it is composed as " extra- 
 branchials" to distinguish them from the true branchial arches of 
 the Gnathostomata. 
 
74 
 
 COMPARATIVE ANATOMY 
 
 In Myxine, the extra-branchial basket-work is quite rudimentary and 
 amongst other peculiarities, the long nasal passage is surrounded by 
 cartilaginous rings, and communicates with the pharynx by a naso-palatine 
 duct. 
 
 No fossil Cyclostomes are known, but Palceospondylns gunni from the Old 
 Red Sandstone of Caithness possibly shows affinities with this group. 
 
 In the Elasmobranchii and Holocephali the skull presents 
 the simplest conditions and most easily comprehensible relations, 
 so that it may be taken as the starting-point for the study of the 
 skull of all other Vertebrates. It consists of a simple carti- 
 laginous and fibrous capsule either more or less immovably united 
 with the vertebral column (Squalida?,) or connected with it by 
 articulations only (Raiidse and Holocephali). 
 
 ejc.br 
 
 Ay.cn, 
 
 FIG. 55. SKULL OF DOGFISH (Scyllium canicula}. (From T. J. Parker's 
 Biology, after W. K. Parker.) 
 
 Cr. cranium ; aud.cp. auditory capsule; or. orbit; olf.cp. olfactory capsule ; r. 
 rostral cartilage ; hy.m. hyomandibular ; up.j. palatoquadrate ; l.j. Meckel's 
 cartilage ; hy. en. ventral part of hyoid arch ; Ig. Iff. ligaments supporting the 
 jaws from the cranium; lb. labial cartilage; br.a. 1 5, branchial arches; 
 br.r, br.r', branchial rays arising from the hyoid and branchial arches ; ex. 
 br. extra-branchial cartilages ; Nv. 2, optic foramen ; Nv. 5, foramen for 
 trigeminal and facial nerves. (The spiracular cartilage is not indicated.) 
 
 True bones are never developed, the cartilage being merely 
 calcified. In Elasmobranchs the palatoquadrate and lower jaw 
 are provided with numerous teeth, arranged in rows ; in the 
 Holocephali, the teeth have the form of strong and sharp-edged 
 plates. 
 
 The nasal region is often elongated to form a long cut- water or 
 rostrum (intertrabecula), at the proximal end of which the olfactory 
 sacs are situated, their cavities being separated from the cranial 
 cavity by a fibrous membrane (''lamina cribrosa"). Behind them 
 are the deep orbital hollows, which are bounded posteriorly by the 
 
THE SKULL 75 
 
 strongly projecting auditory regions. Labial cartilages are present 
 in connection with the lips, nostrils, and jaws (Figs. 55, 56, and 57). 
 
 The palatoquadrate is usually only united to the basis cranii by 
 ligaments, but in the Chimseroids (Fig. 57) it becomes immovably 
 fused with it, whence their name of Holocephali. In the Sharks 
 and Rays the palatoquadrate is not directly united to the skull, but 
 is suspended from it by the hyomandibular (p. 70, Figs. 55 and 56). 
 In this case the skull may be described as liyostylic, to distinguish 
 it from autostylic skulls, in which the hyoid takes no part in 
 the suspensorium (Fig. 57). A cleft, the spiracle, is situated in 
 front of the hyomandibular, and leads into the cavity of the mouth ; 
 on its anterior wall may be found remnants of the embryonic 
 spiracular gill, beneath which is a spiracular cartilage (comp. p. 70, 
 and Fig. 5(>). 
 
 The branchial skeleton is always well developed, and owing 
 to secondary segmentation and fusion of its parts exhibits char- 
 
 FIG. 56. SKULL OF SKATE. (After W. K. Parker.) 
 
 An, auditory capsule ; Na, olfactory capsule ; P.N, prenasal rostrum ; Pt.Pt, Qu, 
 palatoquadrate bar ; Mck, mandibular (Meek el's) cartilage ; M.Pt, spiracular 
 cartilage ; H.M, hyomandibular ; i.h.I, interhyal ligament ; E.Hy, epihyal ; 
 C.Hy, ceratohyal ; FT.Hy, hypohyal ; H.Br, 1, 2, J, hypobranchials ; abov r e 
 them are seen the cerato-, epi-, and pharyngo-branchiais ; //, optic foramen ; 
 V, foramen for trigeminal and facial nerves. (The branchial rays and extra- 
 branchial s are not indicated.) 
 
 acteristic modifications. On the outer circumference of each 
 branchial arch, as well as on the hyomandibular and hyoid, radially- 
 .arranged cartilaginous rays are situated, which serve as supports 
 for the gill-sacs (Fig. 55). Externally to these rays small rod-like 
 " extra-branchial " cartilages are present. 
 
 In nearly all Elasrnobranchs the gill-slits open freely on to 
 the surface of the body, but in Chlamydoselache and the Holo- 
 cephali a fold of skin arising from the hinder border of the 
 hyomandibular overlies them. This is the first indication of a gill- 
 <cover or operculum, such as is present in Teleosts and Ganoids. 
 
 Amongst the Ganoids, the lowest condition is met with in 
 
76 
 
 COMPARATIVE ANATOMY 
 
 those forms in which the hyaline primordial skull, immovably fixed 
 to the vertebral column, is still retained (Fig. 58). These forms 
 are spoken of as Cartilaginous Ganoids. As in Elasmobranchs, 
 the cranial cavity reaches forwards to the ethmoidal region, but is 
 separated from the latter by cartilage. The appearance of definite 
 bones, however, divides them sharply off from the Elasmobranchs, 
 and proves their skull to be at a much higher stage of develop- 
 ment. These bones have the form of richly sculptured plates and 
 shields, and are developed partly from the mucous membrane 
 lining the mouth and covering the visceral skeleton, and partly 
 from the skin covering the roof of the skull. In the first-named 
 
 L O.S 
 
 FIG. 57. SKULL or Chimara monstrosa, LATERAL VIEW. (From 
 Parker and Haswell's Zoology, after Hubrecht. ) 
 
 a.s.c. position of anterior semicircular canal; c.hy. ceratohyal ; ep.hy. epi- 
 hyal ; fr.cl. frontal clasper ; Ji.s.c. position of horizontal semicircular canal ; 
 i.o.s. interorbital septum; Ib. 1, Ib. 2, Ib. 3, labial cartilages; McLC. 
 mandible ; Nv. 3, optic foramen ; Nv. 10, vagus foramen ; olf. cp. olfactory 
 capsule; op.r. opercular rays; pcd.qu. palatoquadrate ; ph.hy. pharyngo- 
 hyal ; p.s.c. position of posterior semicircular canal; qu. quadrate region; 
 r. rostrum. 
 
 region a narrow parasphenoid forms a roof to the oral cavity 
 and extends for some distance along the ventral side of the 
 vertebral column. Ali- and orbito-sphenoids are present in 
 the walls of the brain-case. The operculum is more pronounced 
 than in the Holocephali, and is also supported by bones. The 
 whole palato-mandibular apparatus, which is comparatively small 
 and in connection with which bones are formed, is connected very 
 loosely with the skull by means of a hyomandibular and sym- 
 plectic, as well as by ligaments (Fig. 58). 
 
 The dermal skeleton attains a much more considerable develop- 
 
THE SKULL 
 
 77 
 
 ment in a second group of these Fishes the Bony Ganoids 
 and gives rise to a dense armour composed of numerous bones 
 lying on the roof and extending into all parts of the skull and 
 jaws (Fig. 59j. The cartilage thus becomes reduced : it is, however, 
 largely retained in Amia. The opercular bones are more highly 
 developed than in cartilaginous Ganoids, and consist of an oper- 
 culum, a preoperculum, a suboperculum, and an interoperculum. 
 
 If all the membrane bones are removed and the cranium separated from 
 the vertebral elements which are fused with it, a surprising similarity will be 
 seen between the skull of Polypterus and that of Elasmobranchs more par- 
 ticularly that of Chlamydoselache and Notidanus. On the other hand, the chon- 
 drocranium of Polypterus shows certain resemblances to that of the Amphibia. 
 
 Fi. 58. CRANIAL SKELETON OF STURGEON (Acipenser) AFTER REMOVAL OF THE 
 
 EXOSKELETAL PARTS. 
 
 WS, vertebral column ; SpN, apertures for spinal nerves ; Psp, neural spines ; 
 Ob, neural arches ; C, notochord; GK, auditory capsule; PF, AF, postor- 
 
 mandible ; De, dentary ; Ar, articular ; ffm, hyomandibular ; Sy, symplectic ; 
 Ih, interhyal ; hy, hyoid ; / to V, first to fifth branchial arches, with their 
 
 segments the double pharyngo-branchial (a), the epibranchial (b) the cerato- 
 branchial (<), and the hypobranchial (d) ; Cop, basal elements of the visceral 
 skeleton ; Ri, ribs. 
 
 The branchial skeleton in Ganoids consists of four or five more 
 or less strongly ossified gill-arches, decreasing in size antero- 
 posteriorly (Fig. 58) ; and in bony Ganoids the surface which looks 
 towards the throat is beset with teeth. 
 
 The Ganoidei are of special interest, as they, with the Elasmobranchii, 
 constitute the entire Fish-fauna through the Silurian, Devonian, and Carbo- 
 niferous periods, and as the Teleostei which appear later, are doubtless derived 
 from them. They also show connection with the Dipnoi and with the oldest 
 Amphibia from the Carboniferous and Trias (Ganocephala, Stegocephala). 
 
 In the Teleostei, the skull presents a large amount of varia- 
 tion ; its ground-plan, however, may always be derived from that 
 
78 
 
 COMPARATIVE ANATOMY 
 
 of the bony Ganoids, as is best seen by a comparison of the 
 Siluroids with Amia. On the other hand, no relations with the 
 
 Amphibia are observable, 
 and we must consider the 
 whole group of the bony 
 Fishes as a side branch of 
 the piscine phylum. 
 
 Much of the cartilagin- 
 ous primordial skull persists 
 in most Teleostei ; the 
 cranial cavity may either 
 reach between the eyes as 
 far as the ethmoid al region, 
 or it may become narrowed 
 and arrested in the orbital 
 region (Fig. 50, C), in 
 which ali-, orbito-, and 
 basi-sphenoid ossifications 
 may occur (Fig. 61). The 
 olfactory organs, as in most 
 other Fishes, consist of two 
 sacs lying in the cartilage of 
 the ethmoidal region. 
 
 The palatoquadrate bar 
 becomes differentiated into 
 a row of bony plates 
 the quadrate, meso- and 
 metapterygoid, pterygoid, 
 and palatine. The audi- 
 tory capsule ossifies from 
 five centres (see p. 72), 
 and in the occipital region, 
 as well as on the dor- 
 sal surface of the skull, 
 numerous bones are de- 
 Pmx, premaxilla ; Na, external nostril ; N t veloped, for details of which 
 
 nasal; Sb, Sb L , anterior and posterior the reader is referred to 
 
 suborbital ; Orb, orbit ; M, maxilla ; 
 
 ftp, spiracular bones ; PO, preopercu- 
 
 lum (?) ; SO, suboperculum ; Op, oper- 
 
 culum ; F, frontal ; P, parietal ; a, b, 
 
 c, d, supraoccipital shields. The two 
 
 arrows pointing downwards under the 
 
 spiracular shields show the position of 
 
 the openings of the spiracles on to the 
 
 outer surface of the skull. 
 
 FIG. 59. SKULL OF Polypterus bichir FROM 
 THE DORSAL SIDE. 
 
 Figs. 60 and 61. 
 
 In many Teleosts a canal, 
 lying in the axis of the base of 
 the skull, encloses the eye- 
 muscles, and opens on either 
 side into the orbit. 
 
 All the bones bounding the oral cavity, viz., the vomer, the 
 parasphenoid, the premaxilla, and the maxilla, may bear teeth. 
 The maxilla, however, is usually edentulous, and both it and the 
 
THE SKULL 
 
 79 
 
 premaxilla vary much as to their development : the latter may even 
 be absent. 
 
 Besides the above-mentioned bones in connection with the 
 palatoquadrate bar, the cranial capsule of Teleosts is sur- 
 rounded by other outworks consisting of bony plates and bars. 
 These arise as true dermal bones in the region of the eyes (orbital 
 ring), and in the gill-covers (opercular bones) : the latter are similar 
 in number and name to those of bony Ganoids. A large number of 
 
 <rtli 
 
 ,. epiot 
 
 dent 
 
 FIG. 60. CRANIAL SKELETON OF THE SALMON. (From the left side.) 
 
 Pmx, premaxilla ; eth, supraethmoid ; nets, nasal ; mx, maxilla ; jug, jugal ; 
 pt, pterygoid ; mpt, mesopterygoid ; mtpt, metapterygoid ; Quad, quadrate ; 
 hyom, hyomandibular ; pal, palatine ; fr, frontal ; o, o, o, o, orbital 
 ring ; par, parietal ; sphot, sphenotic ; epiot, epiotic ; pter, pterotic ; socc, 
 supraoccipital ; op, operculum ; pra>op, preoperculum ; intop, interoperculum ; 
 sttbop suboperculum ; branchiost, branchiostegal rays ; dent, dentary ; art 
 articular ; Zunge, tongue. 
 
 branchiostegal rays are developed in the ventral part of the oper- 
 cular fold, or branchiostegal membrane (Fig. 60). 
 
 Anteriorly, the opercular apparatus lies against a bony chain 
 consisting of three pieces the hyomandibular, symplectic, and 
 quadrate which serves as a suspensorial apparatus for the lower 
 jaw (Fig. 60). The latter consists of Meckel's cartilage and of 
 several bony elements, the largest of which is the dentary: 
 
COMPARATIVE ANATOMY 
 
 lasph 
 
 FIG. 61. A. CRANIAL SKELETON OF SALMON AFTER REMOVAL OF THE JAWS, 
 
 AND ORBITAL AND OPERCULAR BONES. (From the right side.) 
 B. Longitudinal section of the same. The cartilaginous parts are dotted. 
 
 vo, vomer ; p-sph, parasphenoid ; fr, frontal ; ekteth, ectoethmoid ; socc, supra- 
 occipital ; exocc, exoccipital ; basocc, basioccipital ; CoLvert, point of connec- 
 tion of the skull with the vertebral column ; basph, basisphenoid ; orbxph, 
 orbitosphenoid ; alxph, alisphenoid ; epiot, epiotic ; pfero, pterotic ; opisth, 
 opisthotic; proot, prootic ; sphot, sphenotic ; N.olf, canal for the olfactory 
 
 the others are, the articular, angular, and coronoid. The last two, 
 however, may be wanting. 
 
 The hyoid arch is followed by four branchial arches and a 
 rudimentary fifth which forms the "inferior pharyngeal bone." 
 
THE SKULL 81 
 
 The dorsal segments of these arches become fused together to 
 form the "superior pharyngeal bone," which, like the inferior 
 pharyngeal, usually bears teeth. 
 
 A curious asymmetry is seen in the head of adult Pleuronectidce. When 
 hatched, these Fishes are quite symmetrical, but later on the eye of one side 
 becomes rotated, so that eventually both eyes are situated on the same side ; 
 in consequence of this, the skull also becomes asymmetrical. 
 
 The tactile barbules present on the head of many Fishes (e.g., Siluroids) 
 are supported by skeletal parts. 
 
 u. Dipnoi. 
 
 The skull of the Dipnoi is in a sense intermediate between that 
 of the Holocephali, Ganoidei, and Teleostei, on the one hand, and 
 
 FIG. 62. CRANIAL SKELETON, PECTORAL ARCH, AND ANTERIOR EXTREMITY OF 
 
 Protopterus. 
 
 W, W l , the vertebrae which are fused with the skull, with their neural spines (Psp, 
 P$p l ) ; Occ, exoccipital, with the hypoglossal foramina ; Ob, auditorycapsule ; 
 Tr, trabecular region, with the foramina for the trigeminal and facial nerves ; 
 FP, f roil to-parietal ; Ht, membranous fontanelle, perforated by the optic 
 foramen (//) ; SK, supra-orbital ; SE, supra-ethmoid ; NX, cartilaginous 
 nasal capsule ; AF, antorbital process (the labial cartilage, which has a similar 
 position and direction, is not indicated) ; PQ, palatopterygoid, which converges 
 towards its fellow of the other side at PQ 1 ; Sq, squamosal, covering the 
 quadrate ; A, A 1 , articular, joined to the hyoid (Hy) by a fibrous band () ; 
 D, dentary ; ft, Meckel's cartilage, which is freely exposed, and grows 
 out into prominences ; SL, a, b, teeth ; Op, Op 1 , rudimentary opercular 
 bones ; 7 to V, the five branchial arches ; KR, cranial rib ; LK, MK, 
 lateral and median bony lamellae which ensheathe the cartilage of the 
 pectoral arch (Kn, Kn l ) ; co, fibrous band which binds the upper end of the 
 pectoral arch with the skull ; x, articular head of the pectoral arch, with 
 which the basal segment (b) of the free extremity articulates ; *,*, rudimen- 
 tary lateral rays of the extremity (biserial type) ; 1, 2, 3, the three next seg- 
 ments of the free extremity ; K, external gills. 
 
 G 
 
82 COMPARATIVE ANATOMY 
 
 that of Amphibia on the other. In certain respects, however, it 
 presents special characters in which it differs from that of all 
 these forms. 
 
 The chondrocranium is retained either entirely (Ceratodus) or 
 at any rate to a large extent (Protopterus and Lepidosiren), and 
 the cartilage bones are much less numerous than in Ganoids, 
 exoccipitals only being present (Fig. 62). The cranial cavity 
 extends forwards between the orbits to the ethmoidal region, and 
 the lamina cribrosa is largely cartilaginous. The quadrate, which 
 is covered by a squamosal (which corresponds to the preopercu- 
 lum of Fishes), is fused with the cranium, and the connection 
 between the latter and the strongly ossified palatopterygoid, which 
 unites with its fellow anteriorly, is a very close one. 
 
 The lattice-like cartilaginous nasal capsules are situated right 
 and left of the apex of the snout, close under the skin. As in 
 all the higher Vertebrates, each nasal cavity communicates with 
 the mouth by internal nostrils (choance) as well as with the exterior 
 by the external nostrils, which are, however, covered by the upper 
 lip. The labial cartilages are directly connected with the inter- 
 nasal septum. 
 
 The occipital region is immovably connected with the 
 vertebral column, some of the anterior vertebrae being firmly 
 united with the skull. The teeth, which are sharp and blade- 
 like, are covered with enamel, and are borne on the palatoptery- 
 goid and mandible ; small " vomerine teeth " are also present, 
 though there is no vomer. The gill-covers and branchiostegal 
 rays are greatly reduced, and even the five cartilaginous gill- 
 arches are in a very rudimentary condition in Protopterus and 
 Lepidosiren. 
 
 The strong lower jaw is ossified by an articular, a dentary, an 
 angular, and a splenial, on the last mentioned of which the teeth 
 are borne ; Meckel's cartilage extends for a short distance an- 
 teriorly to the dentary. 
 
 The Dipnoi are an extremely ancient group ; they existed in the Trias and 
 Carboniferous periods, and possibly even extended into the Silurian. Several 
 facts as regards their skull cannot be satisfactorily elucidated until something is 
 known of its development. The morphology of the so-called "cranial rib " 
 (Fig. 62), for instance, is not at present understood. 
 
 c. Amphibia. 
 
 Urodela. The comparatively simple skull of tailed Amphi- 
 bians is distinguished from that of bony Fishes in general 
 principally by negative characters, on the one hand by the 
 presence of less cartilage in the adult, and on the other by 
 a reduction in the number of bones. In the larval condition 
 (Fig. 63), the chondrocranium, with its nasal, orbital, and auditory 
 
THE SKULL 
 
 83 
 
 Pmr 
 
 P,n.r To I 
 
 M 
 
 Core 
 
 FIG. 63. SKULL OF A YOUNG 
 AXOLOTL. Ventral view. 
 
 Cdcc Osp 
 
 FIG. 64. SKULL OF Scdamandra atrcu 
 (ADULT). Dorsal view. 
 
 Ci 
 
 -Ccm 
 
 FIG. 65. SKULL OF Salamandra atra (ADULT). Ventral view. 
 
 Tr, trabecula ; OB, auditory capsule ; Fov, fenestra ovalis, closed on one side by 
 the stapes (St) ; Lgt, ligament between the stapes and suspensorium ; Cocc, 
 occipital condyles ; Bp, cartilaginous basilar plate between the auditory cap- 
 sules ; Osp, dorsal tract of the occipital cartilage ; IN, internasal plate, 
 which extends laterally to form processes (TFand AF) bounding the internal 
 nostrils (Ch) ; NK, nasal capsule ; Can, nasal cavity ; Na, external nostrils ; 
 Fl, foramen for the olfactory nerve ; Z, tongue-like outgrowth (intertrabecula) 
 of the internasal plate, which forms a roof for the internasal cavity ; 
 Qu, quadrate ; Ptc, cartilaginous pterygoid ; Pot, otic process, Ped, pedicle, 
 and Pa, ascending process, of the quadrate ; Ps, paraspheiioid ; Pt, bony 
 pterygoid ; Vo, vomer ; PI, palatine ; Pp, palatine process of maxilla ; Vop, 
 vomero-palatine ; Pmx, premaxilla ; M, maxilla ; Os, sphenethmoid ; As, 
 prootic ; N, nasal ; Pf, prefrontal, perforated at D for the lachrymal duct ; 
 F, frontal ; P, parietal ; Squ, squamosal("paraquadrate," Gaupp) ; //, optic, 
 V, trigeminal, and VII, facial foramina ; Rt, point of entrance of the 
 ophthalmic branch of the fifth nerve into the nasal capsule. 
 
 G 2 
 
84 
 
 COMPARATIVE ANATOMY 
 
 regions, has very distinctly the relations described in the introduc- 
 tion to this chapter. The auditory capsules (Figs. 63 to 65) which 
 are bound together by cartilaginous tracts in the basi- and supra- 
 occipital regions, and generally become strongly ossified later by 
 the exoccipitals and prootics, show a new and important 
 modification as compared with those of Fishes in the presence of 
 an aperture, the fenestra ovalis, on the outer and lower side 
 of each. This fenestra is closed by a cartilaginous plug, the 
 stapedial plate, probably corresponding to a part of the wall of 
 the auditory capsule ; from it a rod-like cartilage or bone, the 
 eolumella auris, corresponding phylogenetically to the upper 
 element of the hyoid arch, extends outwards towards the quadrate 
 in most Urodeles and serves to conduct the sound to the "inner 
 ear, the position of the semicircular canals of which is indicated 
 by corresponding cartilaginous ridges on the capsule. 
 
 In all Amphibians two condyles for articulation with the first 
 vertebra are developed on the ventral periphery of the foramen 
 
 mk 
 
 FIG. 66. SKULL AND VISCERAL ARCHES OF Menopoma. (From the side.) 
 
 t, mandible ; II, hyoid ; III- VI, branchial arches ; qu, quadrate, above which 
 is the squamosal ; ar, articular ; mk, Meckel's cartilage, enclosed by the 
 dentary bone. 
 
 magnum. The occipital region is ossified by two exoccipitals, a 
 bony supra- and basioccipital rarely being present in recent 
 forms (certain Anura). 
 
 The large nasal capsules, consisting throughout life of consider- 
 able cartilaginous portions, are connected with the auditory 
 capsules by means of the trabeculae, which give rise to the side 
 walls of the skull and become more or less entirely ossified as 
 the sphenethmoid and prootics. The cranial cavity is closed 
 dorsally by the frontals and parietals, and ventrally by the 
 parasphenoid, which is sometimes provided with teeth. In 
 front of it are the vomers, which bound the internal nostrils ; in 
 adults each vomer becomes fused with the corresponding palatine, 
 which forms a delicate bar lying on the ventral surface of the 
 
THE SKULL 85 
 
 parasphenoid. These relations are secondary, for in the larval 
 condition a typical pal atoquad rate or pterygopalatine bar is present 
 (Fig. 63). The' lamina cribrosa (p. 74) is either cartilaginous (e.g., 
 Salamandra) or membranous (e.g., Triton) ; or the cranial cavity 
 may be closed in front by special modifications of the frontals. 
 
 On the outer side of the vomer lies the maxilla, and in front of 
 this is a premaxilla which usually encloses, or at least bounds, a 
 cavity. The latter bone extends on to the dorsal surface of the 
 skull and abuts against the nasai, behind which usually follows a 
 prefrontal. 
 
 The suspensorium is much more simple than that of Fishes 
 (Figs. 63 66). It consists of the quadrate only, which has 
 usually four typical processes connecting it with surrounding parts, 
 and which becomes fused secondarily with the skull. On the 
 outer surface of the quadrate an investing bone, the squamosal, 1 
 becomes developed. 
 
 In Tylototritoii verrucosus the quadrate sends forwards a process which 
 connects it with the maxilla : this is quite exceptional amongst Urodeles. 
 
 With the exception of the lower jaw, in connection with which 
 articular, splenial, and dentary bones are developed, the visceral 
 skeleton of Urodeles undergoes various modifications in the different 
 types. We may consider the ground-form, as exhibited in the larva, 
 to consist of five pairs of bars in addition to the mandibular arch 
 (Fig. 66). The anterior bar, or hyoid, consists of two segments (Fig. 
 67, A), as do also the two first branchial arches. The third and 
 fourth branchial arches are much smaller, and each is composed of 
 a single segment. All the above-named arches are connected 
 with their fellows of the other side by means of a single or double 
 basal piece. At the close of larval life, that is, when the gills are 
 lost, the two hinder pairs of arches disappear entirely, while the 
 two anterior pairs undergo changes as regards form and position, 
 and may become more or less densely ossified (Fig. 67, B D). 
 
 In the genus Spelerpes, which possesses a sliiig-like tongue, the dorsal 
 segment of the first branchial arch grows out into a long cartilaginous fila- 
 ment, which extends far back under the dorsal integument (Fig. 67, D). 
 
 The skull of the G-ymnophiona differs from that of Urodeles mainly in its 
 extremely solid and strong character, the ossifications being more extensive. 
 
 In the extinct tailed Amphibians (i.e., Stegocephala, Fig. 68) some of which 
 were comparatively gigantic, the cranial bones were very numerous and dense. 
 A parietal foramen was present, as well as a ring of orbital bones. These 
 forms possessed the same number of visceral arches as Urodeles, and it has 
 been shown that they (e.g., Branchiosaurus) underwent a metamorphosis. 
 Existing Amphibia cannot have been derived directly from them. 
 
 Anura. The skull of the tailless Batrachia is at first sight 
 very similar to that of Urodeles. It undergoes, however, an 
 
 1 According to Gaupp, a true squamosal is never present in existing Amphibia, 
 and the bone which is usually so designated he calls the paraquadrate. 
 
86 
 
 COMPARATIVE ANATOMY 
 
 essentially different and much more complicated development, 
 and cannot in any way be directly derived from that of tailed 
 Amphibians. 
 
 flpbrl 
 
 K2T 
 
 -KeJT 
 
 Ktff, 
 
 FIG. 67. HYOBRANCHIAL APPAKATUS OF URODELES. A, Axolotl (Siredon stage 
 of Amblystoma) ; B, Salamandra maculota; C, Triton cristatus; D, Spelerpes 
 
 fuscus. 
 
 Bbr, I, II, first and second basibranchial ; Ke.ff, ceratohyal ; HpH, hypohyal ; 
 Kebr I, II, first and second ceratobranchial ; Epbr I to IV, first to fourth 
 epibranchial ; KH, KH } , small anterior and posterior pairs of cornua ; 
 O.th, part of skeleton of larynx ; G.th, thyroid gland. 
 
 A suctorial mouth, provided with labial cartilages and horny 
 jaws, is present in the larva. An advance on Urodeles is seen in 
 the formation of a tympanic cavity which is closed externally by a 
 tympanic membrane, while internally it opens into the mouth by an 
 
THE SKULL 
 
 87 
 
 Eustachian aperture. With the exception of certain small regions 
 (fenestrae) on the dorsal side, the skull of Anura forms a com- 
 
 FIG. 68. RESTORATION OF THE SKULL OF A STEGOCEPHALAN (from the 
 Carboniferous of Bohemia). (After Fritsch.) 
 
 Pmx, premaxilla ; J/, maxilla ; N, nasal ; Na, nostril ; F, frontal ; Pf, prefrontal ; 
 P, parietal ; Fp, parietal foramen ; Socc, supraoccipital ; Br, branchial 
 apparatus ; Oc, sclerotic ring (orbital bones. ) \ 
 
 plete cartilaginous box, the ethmoid region being at first entirely 
 cartilaginous, and later becoming ossified by a sphenethmoid, which 
 
 PP 
 
 occ 
 
 FIG. 69. SKULL OF Rana esculenta. Ventral view. (After Ecker. ) 
 The investing bones are removed on the right side. 
 
 Coc.c, occipital condyles ; Olat, exoccipital ; GK, auditory capsule ; Qu, quadrate ; 
 Qjg, quadratojugal ; Pro, prootic ; Ps, parasphenoid ; As, alisphenoid region ; 
 Pt, bony pterygoid ; PP, palatopterygoid ; FP, frontoparietal ; E, spheneth- 
 moid (girdle bone) ; Pal, palatine ; Vo, vomer ; M, maxilla ; Pmx, premaxilla ; 
 A", A n , cartilages in connection with the nasal capsules; W.K, prorhinal 
 cartilage ; //, V, VI, foramina for optic, trigeminal, and abducent nerves. 
 
88 
 
 COMPARATIVE ANATOMY 
 
 encircles the whole skull in this region and is perforated by the 
 olfactory nerves. 
 
 In the adult the bones are not so numerous as in Urodeles, and 
 the frontal and parietal of either side as a rule fuse together, thus 
 giving rise to a fronto-parietal. The maxillary bar grows back- 
 wards much further than in Urodeles, and becomes connected with 
 the suspensorium by means of a small intermediate bone, the quad- 
 ratojugal (Fig. 69). The maxillary arch is therefore complete, as 
 
 in Tylototriton amongst 
 Urodeles (p. 85). The 
 palatoquadrate is united 
 anteriorly with the carti- 
 laginous nasal capsule. 
 (For the relations of 
 the bones bounding the 
 mouth-cavity compare 
 Fig.69.) The bones of the 
 lower jaw are a dentary 
 and an angular, the distal 
 end of Meckel's cartil- 
 age ossifying as a small 
 " mentomeckelian." 
 
 There is a much 
 greater reduction of the 
 branchial skeleton at the 
 close of larval life than 
 in Urodeles. In. the 
 larva representatives of 
 the hyoid and of four 
 branchial arches can be 
 recognised, but these are 
 all fused together and 
 form a continuous struc- 
 ture, reminding one of 
 the branchial basket- 
 work of the Lamprey. 
 In the adult this be- 
 comes greatly reduced, 
 and the apparatus con- 
 sists of a broad cartilaginous plate in the floor of the mouth, with 
 long anterior and shorter posterior (thyro-hyal) cornua, the latter 
 of which become ossified. 
 
 D. Reptiles. 
 
 Although as regards the structure of the skull existing Reptiles 
 and Amphibians are widely separated from one another, certain 
 resemblances exist between their extinct representatives (e.g., 
 PalaBohatteria and the Stegocephala). 
 
 ct.c. 
 
 FIG. 70. HYOBRANCHIAL SKELETON OF LARVAL 
 (A) AND ADULT (B) FROG. 
 
 (After Gaupp. ) 
 
 bs, body of the hyoid ; a.c, anterior cornua ; 
 p.c, posterior cornua. 
 
THE SKULL 
 
 89 
 
 Excepting in the naso-ethmoidal region, the whole chondro- 
 cranium usually becomes almost obliterated by an extensive process 
 
 trans 
 
 art 
 
 ilenl 
 
 any 
 
 FIG. 71. SKULL OF Lacerta agilis (from Parker and Haswell's Zoology, 
 after W. K. Parker). 
 
 A, from above ; B, from below ; C, from the side, ang, angular ; art, articular ; 
 bas.oc, basioccipital ; bas.ptg, basipterygoid processes; bas.sph, basi- 
 sphenoid ; col, epipterygoid ; cor, coronary ; dent, dentary ; eth, ethmoid ; 
 ex.oc, exoccipital ; ext.nar, external nares ; for. mag, foramen magnum; 
 fr, frontal ; int.nar, internal nares ; ju, jugal ; Icr, lachrymal ; max, maxilla ; 
 nas, nasal; oc.cond, occipital condyle ; olf, olfactory capsule; opi.ot, opis- 
 thotic ; opt.n, optic nerve; pal, palatine; par, parietal; para, para- 
 sphenoid ; par.f, parietal foramen ; p.mx, premaxillas ; pr.fr, pref rental ; 
 ptg, pterygoid ; pt.orb, postorbital ; qu, quadrate ; s.ang, supra-angular ; 
 s.orb, supraorbitals ; sq, squamosal ; supra.t. 1 , supra.t.-, supratemporals 
 (" paraquadrate," Gaupp) ; trans, transverse bone ; supra.oc, supraoccipital ; 
 vom, vomer. 
 
90 
 
 COMPARATIVE ANATOMY 
 
 of ossification, which gives the skull a very firm and solid appear- 
 ance ; only amongst Lizards (Fig. 71), and especially in Hatteria 
 is the cartilage retained to any considerable extent, and owing to the 
 conformation of the bones in the posterior region, the skull in these 
 forms presents a number of distinct spaces or fossas in the dry state. 
 
 In Snakes and Amphisbsenians the cranial cavity extends 
 forwards between the orbits as far as the ethmoidal region, while 
 in the Lacertilia, Chelonia, and Crocodilia in which a fibro-carti- 
 laginous interorbital septum perforated by the olfactory nerve is 
 present its anterior boundary is much further back. 
 
 The parasphenoid, which plays so important a part as an 
 investing bone of the roof of the mouth in Fishes and Amphibians, 
 
 Eth-Pmv 
 
 FIG. 72. SKULL OF SNAKE (Tropidonotus matrix), dorsal view. 
 FIG. 73. ,, ,, ,, ventral view. 
 
 Cocc, occipital condyle ; Osp, supraoccipital ; 01, exoccipital ; Fov, fenestra ovalis ; 
 Pe, periotic ; P, parietal ; F, frontal ; f" 1 , postfrontal ; Pf, prefrontal ; 
 Eth, ethmoid ; N, nasal ; Pmx, premaxilla; M , maxilla ; Bp, basioccipital ; 
 Bs, basisphenoid ; Ch, posterior nostrils ; Vo, vomer ; PI, palatine ; Pt, 
 pterygoid ; Ts, transverse bone ; Qu, quadrate ; Squ, squamosal ; Art, 
 articular ; Ag, angular ; 3A, supra-angular ; Dt, dentary ; //, optic foramen. 
 
 begins to disappear ; amongst Reptiles it only attains any im- 
 portant development in Snakes, where the anterior part remains 
 and forms the base of the interorbital region. Its place is taken 
 by two cartilage bones, the basioccipital and basispkenoid, situated 
 along the basis cranii. In contradistinction to the Amphibia, only 
 a single condyle connects the skull with the vertebral column : 
 this, on close examination, is seen to be formed of three parts, 
 derived from the basioccipital and exoccipitals respectively. 
 
THE SKULL 91 
 
 The roofing bones of the skull are well-developed and in the 
 Lacertilia may become closely united with overlying dermal bones, 
 while the trabecular region (ali- and orlitosplicnoids) becomes of 
 secondary importance in the adult, its place being partly taken by 
 processes growing downwards from the frontal and parietal : 
 this is especially the case in Snakes. 
 
 The parietals are paired in the Chelonia and in Hatteria ; in 
 all other Keptiles they become fused together, as do also the 
 frontals in many Lizards and Crocodiles. A parietal foramen 1 is 
 present in many Lizards. 
 
 The topographical relations of the several bones to one another 
 are shown in Figs. 71 to 74. It will be seen in them that the 
 ground-plan of the Urodele skull is here fundamentally retained. 
 In addition, however, to a postorbital, an imperfect circumorbital 
 ring of bones is present in Lizards. In many Lizards, moreover, 
 
 "Coee 
 
 FIG. 74. SKULL OF YOUNG WATER- TORTOISE (Emys euroficea}. Side view. 
 
 Osp, supracccipital, which gives rise to a crest ; Pf, prefrontal, which forms a 
 great part of the anterior boundary of the orbit ; /, point of entrance of the 
 olfactory nerve into the nasal capsule ; Na, external nostril ; Si, interorbital 
 septum ; UK, horny sheaths of jaws ; lug, jugal ; Qjg, quadratojugal ("para- 
 quadrate," Gaupp) ; Mt, tympanic membrane ; BP, cartilaginous interval 
 between basioccipital and basisphenoid ; Md, mandible. Other letters as in 
 Figs. 72 and 73. 
 
 a rod-like bone, the cpipterygoicl (also represented in Crocodiles), 
 connects the parietal with the pterygoid, and a transverse lone 
 extending from the maxilla to the pterygoid is typically present 
 in Reptiles, but is wanting in the Chelonia and Typhlopidas. 
 
 The auditory capsules are ossified from three centres, the 
 prootic usually remaining free, and the epiotic uniting with the 
 supraoccipital and the opisthotic with the exoccipital. A fenestra 
 rotunda is present in its walls in addition to a fenestra ovalis, into 
 which latter the stapedial plate of the columella is inserted (see 
 p. 84), and the tympanic cavity in most Reptiles communicates 
 with the pharynx by means of an Eustachian tube. 
 
 1 In certain Chameleons its representative is in the frontal. 
 
92 
 
 COMPARATIVE ANATOMY 
 
 The columella here also probably arises in connection with the upper end 
 of the hyoid arch (see p. 84), with which it is continuous in Hatteria. 
 
 The quadrate alone forms as the suspensorium for the lower 
 jaw : it may be articulated with the skull (Ophidia, 1 most 
 
 . Lacertilia) or firmly fixed to it 
 JC x^X^Hatteria, Chelonia, Crocodilia). 
 
 According to Gaupp, a squamosal is 
 wanting in narrow-mouthed Snakes and 
 Hatteria, and a paraqiiadrate, comparable 
 to^tnat of the Amphibia (p. 85) is present 
 in almost all Lizards and Chelonians, a 
 quadratojugal being found only in Hatteria. 
 
 The pterygopalatine arch is well 
 developed in all Reptiles. In Snakes 
 and Lizards it is more or less movable 
 and free from the base of the skull, 
 while in Chelonians and Crocodiles it 
 meets with its fellow to a greater or 
 less extent in the middle line, and 
 shelf-like palatine processes of the 
 maxilla also come into connection 
 with the palatines : thus a secondary 
 roof is formed to the mouth- cavity 
 distinct from the true (sphenoidal) 
 base of the skull. The cavity thus 
 formed closes in the posterior pro- 
 longation of the nasal chambers, 
 which consequently become sharply 
 differentiated from the mouth. In 
 Chelonians the pterygoid bones do 
 not take part in the formation of this 
 hard palate, which in Crocodiles is 
 much more markedly developed, 
 and is formed by the premaxillas, 
 maxillae, palatines, and pterygoids, 
 the posterior nostrils here opening 
 far back into the pharynx (Fig. 75). 
 
 A number of bones arise in connection with the lower jaw, 
 viz., a dentary, angular, supra-angular, splenial, coronoid, and 
 articular. 
 
 Teeth are well developed in all Reptiles except Chelonians, 
 
 1 In Snakes (Figs. 72 and 73) (except Tortrix), the quadrate is only indirectly 
 connected with the skull by means of the squamosal, which extends backwards, 
 and thus throws the articulation of the lower jaw far back, giving rise to a very 
 wide gape. In most Snakes, and particularly in the Viperine forms, the facial 
 pable of movement upon one another, but in Typhlops they are im- 
 
 FIG. 75. SKULL OF A YOUNG 
 CROCODILE. (Ventral view.) 
 
 Cocc, occipital condyles ; Ob, 
 basioccipital ; Ch, internal 
 nostrils ; Pt, pterygoid ; Orb, 
 orbit ; PI, palatine ; M, 
 palatine process of maxilla ; 
 Pmx, premaxilla ; Ts, trans- 
 verse bone ; Jy, jugal ; Qj, 
 quadratojugal ( ' ' paraquad- 
 rate," Gaupp) ; Qu, quadrate. 
 
 bones are ca 
 
 movably connected with the skulL 
 
 by a more or less elastic ligament. 
 
 The two rami of the mandible are connected 
 
THE SKULL 
 
 93 
 
 KIT 
 
 in which they are replaced functionally by strong horny sheaths 
 on the edges of the jaws. The teeth may be borne on the 
 palatine and pterygoid, as well 
 as on the maxilla, premaxilla 
 (which is usually unpaired), 
 and dentary. 
 
 In the young Hatteria only 
 amongst existing Reptiles do the 
 vomers bear teeth (usually one on 
 each). In certain fossil forms brush - 
 like masses of sphenoidal teeth 
 were present. 
 
 The remarkable horned skull of 
 the gigantic Ceratopsidce (Diiio- 
 sauria) which reached a length of 
 nearly seven feet, possessed horny 
 beaks in addition to teeth oil the 
 maxilla and dentary. A parietal 
 foramen w T as present. 
 
 In correspondence with 
 the absence of branchial re- 
 spiration during development, 
 the branchial apparatus plays 
 no great part in Reptiles, and 
 often only the slightest traces 
 of it are seen : thus in Snakes, 
 for instance, only the hyoid 
 remains, and this not always. 
 In Chelonians a basal piece 
 ("basihyobranchial") as well 
 as the first branchial arch per- 
 sist in addition (Fig. 76). 
 
 FIG. 76. HYOBRANCHIAL APPARATUS 
 WITH LARYNX AXD TRACHEA or Emys 
 europcea. 
 
 ZH, basihyobranchial, which widens at 
 ZB and bears the cricoid (RK) and 
 arytenoid (AK) cartilages of the 
 larynx; KH, lesser hyoicl cornua; 
 ZH, greater hyoid cornua ; IK, first 
 branchial arch ; TV, trachea. 
 
 E. Birds. 
 
 The skull of Birds is formed on a similar plan to that of 
 Reptiles more particularly of Lizards, but it exhibits certain 
 special characteristics (Fig. 77). 
 
 The brain-case is proportionately very large, and all the cranial 
 bones show a tendency to run together by the obliteration of the 
 sutures originally present between them ; they are usually delicate 
 and spongy (" pneumatic "), thus contrasting greatly with those of 
 Reptiles. 1 
 
 Only in the region of the nose does the cartilage persist 
 throughout life to any extent, and even here not always. 
 
 1 It should, however, be mentioned that the development of air spaces 
 within the bones of the skull is hinted at in Crocodiles and certain fossil 
 Reptiles. 
 
94 COMPARATIVE ANATOMY 
 
 The unpaired occipital condyle no longer lies at the posterior 
 boundary of the skull, but becomes relatively shifted forward along 
 its base, so that the axis of the latter lies at an angle with that of 
 the vertebral column. 
 
 The basis cranii is formed by a basioccipital and a basisphenoid, 
 from which latter a bony rostrum, the remains of the anterior part 
 of the parasphenoid, extends forwards. The posterior part of the 
 parasphenoid persists as a large plate, the basitemporal, which 
 underlies the basisphenoid and part of the basioccipital. Above 
 the rostrum a small presphenoid is present in the embryo, and 
 orbitosphenoids and alisphenoids are better developed than in 
 Lizards. The auditory capsules ossify by three centres, and the 
 relations of the tympanic cavity, auditory fenestra3, and columella 
 are very similar to those of Reptiles. The two Eustachian tubes 
 open together in the middle line. 
 
 The quadrate is movable upon the skull, as is also the whole 
 rnaxillopalatine apparatus ; the palatopterygoid bar is separated 
 from its fellow in the middle line and slides on the rostrum of the 
 basisphenoid, thus allowing the beak to be raised or lowered to a 
 greater or less extent : a complete bony palate comparable to that 
 of Crocodiles is consequently never present. This mobility of -the 
 upper jaw is most marked in Parrots, in which the frontonasal joint 
 forms a regular hinge. 
 
 The vomers, which may be absent, usually unite with one 
 another, and with the palatines in a greater or less degree. 1 The 
 posterior nostrils are always situated between the vomers and 
 palatines. The maxilla and quadrate are connected by a jugal 
 and a quadratojugal, and a squamosal is present ; small bones may 
 also occur in the neighbourhood of the lachrymal. (For other 
 details, compare Fig. 77.) 
 
 Teeth were present in Jurassic and Cretaceous Birds (Archa30- 
 pteryx, Hesperornis, Ichthyornis), but are no longer developed in 
 existing forms, their place being taken functionally by horny 
 sheaths covering the bones of the jaws, which thus form a beak, 
 much as in Chelonians. 
 
 Several bones are developed in connection with the lower jaw, 
 the relations of which are essentially similar to those seen in 
 Reptiles : they, however, become fused together in the adult, and 
 the two rami of the mandible unite distally by synostosis. 
 
 The visceral skeleton is greatly reduced, though the basihyal 
 and basibranchial which are embedded in the tongue, as well as 
 the first branchial arch persist, and the latter may, as in the 
 Woodpecker, grow out into a pair of very long jointed rods 
 extending far over the skull. 
 
 1 The differences in details as regards the arrangement of the bones of the 
 palate are important for purposes of classification. 
 
THE SKULL 
 
 95 
 
 FIG. 77. SKULL OF A WILD DUCK (Anas boschas). A, from above ; B, from 
 below ; C, from the side. (From a preparation by W. K. Parker). 
 
 a.l.s, alisphenoid ; ag, angular ; ar, articular ; a.p.f, anterior palatine foramen ; 
 b.t, basitemporal ; b.o, basioccipital ; b.pg, basipterygoid ; b.s, basisphenoid ; 
 d, dentary ; e.n, external nostrils; eth, ethmoid; e.o, exoccipital ; e.u, 
 Eustachian aperture ; fr, frontal ; f.m, foramen magnum ; i.c, foramen for 
 internal carotid artery ; j, jugal ; Ic, lachrymal ; mx.p, maxillopalatine 
 process ; mx, maxilla ; n, nasal ; n.px, nasal process of the premaxilla ; px, 
 premaxilla ; p, parietal ; ps, presphenoid ; pg, pterygoid ; pi, palatine ; p.n, 
 internal nostrils; q, quadrate; q.j, quadratojugal ; ?q, squamosal ; s.o, 
 supraoccipital ; ty, tympanic cavity ; v, vomer ; //, foramen for optic nerve ; 
 F, for trigeminal ; IX, X, for glossopharyngeal and vagus ; XII, for 
 hypoglossal. 
 
96 
 
 COMPARATIVE ANATOMY 
 
 F. Mammals. 
 
 In Mammals there is a much closer connection between the 
 cranial and visceral regions of the skull than is the case in the 
 Vertebrates already described. In the fully-developed skull 
 both maxillary and palatopterygoid regions are united to 
 
 FIG. 78 A. LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF A, 
 Salamandra maculosa, B, Testudo grcKca, AND C, Corvus corone, TO SHOW THE 
 RELATIONS BETWEEN THE CRANIAL AND VISCERAL PORTIONS. 
 
 the cranium, though a facial and a cranial region can still be 
 distinguished. The higher we pass in the Mammalian series, 
 the more does the former come to lie below instead of in front of 
 the latter. In Man the facial skeleton is proportionately small 
 
THE SKULL 
 
 97 
 
 when contrasted with the large cranial portion of the skull, 
 and the reduction of the angle between the basi-cranial and 
 
 A 
 
 FIG. 78 B. LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF A, DEER, 
 B, BABOON, AND C, MAN, TO SHOW THE RELATIONS BETWEEN THE CRANIAL 
 AND VISCERAL PORTIONS. 
 
 vertebral axes is carried still further than in Birds (comp. Fi<?s. 
 78 A and B). 
 
 The base of the skull is mainly preformed in cartilage, as in 
 Keptiles and Birds. The parasphenoid has practically disappeared, 
 
 H 
 
98 
 
 i/vw' 
 J 
 
 ^; 
 
 # 
 
 ,s>// 
 
 ^+^fv 
 
 , l .X5 ^ 
 
 <t*f 
 
 -y 
 
 ,ff. 
 
 Jot 
 
 SU 
 
THE SKULL 99 
 
 FIG. 79. SKULL or GREYHOUND. A, from above; B, from the side; C, from 
 below ; and D, in longitudinal section. 
 
 Jm, premaxilla ; N, nasal ; M, maxilla, with the infraorbital foramen (Finf) ; Jg y 
 jugal ; Sq, squamosal ; Pjt, zygomatic process of the squamosal ; L, lachry- 
 mal, surrounding the lachrymal canal ; P, parietal; Sq.occ, supraoccipital : 
 C.occ, occipital condyles, on the exoccipitals ; B.occ, Occ.bas, basioccipital ; 
 Pal (P in C), palatine ; Pt, pterygoid ; Sph, alisphenoid ; Sph', basisphenoid ; 
 Sph", presphenoid ; Maud, external auditory meatus ; T, tympanic ; For.m, 
 foramen magnum ; Pet, petrous portion of periotic ; Cho, posterior narial 
 passage ; Vo, vomer ; Eth, lamina perpendicularis of the ethmoid ; Eth\ 
 cribriform plate ; Cav.yl, gleiioid cavity for the lower jaw. 
 
 the anterior part of the basis cranii being formed by the ossification 
 of the cartilage : this either gives rise to a distinct presphenoid 
 (Marsupials, Rodents, and some Insectivores), or may be due 
 to a union of the basal parts of the two orbitosphenoids. Ali- 
 sphenoids, as well as a basisphenoid, a basioccipital, a supraoccipital, 
 and exoccipitals are always present, the paired condyles being 
 furnished by the exoccipitals (Fig. 79). The cranial cavity is 
 closed in anteriorly by the bony lamina cribrosa or cribriform 
 plate of the ethmoid, which has numerous perforations for the 
 olfactory nerves in all Mammals but Ornithorhynchus. 
 
 The auditory capsules are ossified from prootic, epiotic, 
 and opisthotic centres, which early fuse together to form the 
 periotic or petromastoid bone. The denser internal (petrous) 
 portion of this encloses the essential part of the organ of hearing, 
 and a fenestra- ovalis and fenestra rotunda are present on its outer 
 surface : the more spongy mastoid portion reaches the surface of 
 the skull between the exoccipital and the tympanic bone. 1 The 
 latter overlies the petrous portion of the periotic, and gives 
 attachment to the tympanic membrane : in the Placentalia it 
 forms the tubular external auditory passage or meatus, below 
 which it usually expands into a bulla tympani, which encloses 
 the tympanic cavity and communicates with the pharynx 
 by means of the Eustachian tube. The "temporal bone" of 
 human anatomy represents the fused periotic, tympanic, and 
 squamosal. 
 
 The cranial cavity is roofed in by frontals, parietals, an inter- 
 parietal, and a supraoccipital : these, like many of the other cranial 
 bones, are united by sutures which usually persist, at any rate for 
 a long time. Many of the bones are more or less spongy internally, 
 and may contain definite air-sinuses. 
 
 Most of the true Ruminants are provided with horns projecting from the 
 frontal bones ; these are of three kinds : 
 
 In the Cbrieproid (Bovinse, Antelopinse, Caprime, Ovinae) hollow bony pro- 
 cesses are developed from the frontals, which become enveloped by horn formed 
 from the epidermis. In the Cervidce, a solid membrane bone becomes developed 
 in the clermis round each process of the frontal, with which it fuses. This 
 grows out to form the antler, and after attaining its full development, the 
 skin covering it dries up owing to the development of the "burr" at its base ; 
 
 1 According to Gaupp, the tympanic corresponds to the " paraquadrate " of 
 Amphibia and Reptilia (pp. 85 and 92). 
 
 H 2 
 
100 COMPARATIVE ANATOMY 
 
 this constricts the vessels, and the antler, being deprived of nutriment, falls 
 off periodically at the close of the breeding season. In the young animal, 
 the antlers are simple, but year by year they become more complicated and 
 branched. Giraffes possess persistent antlers covered by hair .and without any 
 process from the frontal : they do not become anchylosed to the latter bone. 
 The differentiation into horned and antlered forms first began in the 
 Miocene epoch, 
 
 In the nasal cavity, scroll-like turbinals l are present : these are 
 preformed in cartilage, and unite with the surrounding bones. The 
 two nasal chambers are separated from one another by a cartilaginous 
 septum nasi, the posterior part of which becomes ossified by a 
 vertical plate (mesethmoid) in connection with the lamina cribrosa. 
 The vomer, which is unpaired in the adult, is situated immediately 
 below the nasal septum. Cartilage is retained only in the latter 
 region and around the external nostrils (" aliseptal " and " alinasal 
 cartilages "). The nasal cavities communicate anteriorly with the 
 mouth by means of the incisive or naso-palatine canals as well as 
 posteriorly by the internal nostrils. 
 
 As regards the structure of the hard palate, Mammals 
 agree essentially with Crocodiles, but the small pterygoids (except, 
 e.g., in Anteaters and some Cetaceans) do not take part in its 
 formation. The palate is very long in Echidna and in certain 
 Edentata and Cetacea, and often (e.g., Marsupialia) presents 
 unossified vacuities. 
 
 The premaxilla takes an important part in .enclosing the 
 nasal cavity : it also contributes to the hard palate, and surrounds 
 the nasopalatine canal. In the lateral parts of the face of most 
 Mammals, the jugal or malar connects the maxilla with a process 
 of the squarnosal (instead of with the quadrate, as in Amphibia 
 and Sauropsida) : thus a zygomatic arch is formed from these three 
 bones (Fig. 79). In most cases (e.g., Ungulata and Primates) the 
 jugal is also connected with a process of the frontal, and thus the 
 orbit becomes more or less completely separated from the temporal 
 fossa. 
 
 The tympanic membrane is connected with the membrane of 
 the fenestra ovalis by an articulated chain of small auditory 
 ossicles, extending across the tympanic cavity and consisting of the 
 malleus, incus (with its orbicular apophysis), and stapes instead of 
 by a single columella as in Amphibians and Reptiles. The two 
 former of these bones arise in the embryo from the proximal 
 end of the mandibular arch, one portion of which becomes 
 constricted off to form the incus and another the malleus, 
 both portions afterwards becoming ossified. The part of the arch 
 in the lower jaw, distal to the malleus, corresponds to MeckeFs 
 cartilage, and in Fig. 80 the two are seen still in continuity. 
 The stapes, which is stirrup-shaped in all Mammals but Mono- 
 tremes and certain Marsupials and Edentates, plugs the fenestra 
 
 1 For details of the turbinals in Mammals and otlier Vertebrates, compare the 
 section on the olfactory organ. 
 
THE SKULL 
 
 ior 
 
 ovalis on the one hand and articulates with the incus on 'the 
 other, while the malleus articulates with the incus, and its 
 manubrial process is attached to the tympanic membrane. The 
 above facts indicate that the malleus corresponds to the articular 
 element of the mandible of lower Vertebrates, and the incus to 
 the quadrate. 
 
 The morphology of the stapes is not by any means clear ; phylogenetically 
 it certainly corresponds to the upper end of the hyoid arch (pharyngo-hyal or 
 hyomandibular of Fishes), but its homology with this element has not been 
 proved ontogenetically. Its basal plate, however, doubtless corresponds to 
 the stapedial plate of Amphibia and Sauropsida (comp. pp. 84 and 92). 
 
 e.n 
 
 oc.e 
 
 FIG. 80. SKULL OF EMBRYO OF ARMADILLO (Tatusia hybrida). (Modified from a 
 drawing by W. K. Parker. ) 
 
 a.fy, tympanic annulus ; au, auditory capsule ; b.hy, basihyal ; c.hy, ceratohyal ; 
 cr : cricoid ; d, dentary ; e.hy, epihyal ; e.n, external nostril ; eo, exoccipital ; 
 f, frontal; h.hy, hypohyal ; i, jugal ; in, incus ; Ic, lachrymal ; mk, Meckel's 
 cartilage; ml, malleus; mx, maxilla; n, nasal; oc.c, occipital condyle ; p, 
 parietal ; jm, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ; s.t, 
 superior turbinal ; st.m, stapedius muscle ; $q, squamosal ; th, thyroid ; tr, 
 trachea ; F 1 , V 2 , foramina through which the first and second divisions of 
 the trigeminal pass out from the orbit ; //, optic foramen. 
 
 Thus the parts of the mandibular arch which form the hinge of 
 the jaw in lower Vertebrates are in Mammals utilised to conduct 
 sound-vibrations to the internal ear. Around Meckel's cartilage 
 a membrane-bone is developed corresponding essentially to the 
 dentary ; this forms the entire lower jaw of the adult and develops 
 a new articulation with the squamosal : this arrangement is 
 characteristic of and confined to the Mammalia (Figs. 79 and 80). 
 The two rami of the mandible usually unite distally. 
 
 Teeth, which are only exceptionally wanting (e.g., Echidna. 
 
1 102 COMPARATIVE ANATOMY 
 
 'certain Edentates), are confined to the premaxilla, maxilla, and 
 mandible. 
 
 The hyoid arch (Fig. 80) is connected proximally with the 
 auditory capsule and distally with the base of the third visceral 
 (that is, the first true branchial) arch. It becomes more or 
 less ossified, but the greater part is usually reduced to a fibrous 
 band, and may be quite rudimentary; its proximal end forms 
 the styloid process of the periotic, and its distal the lesser 
 (anterior) cornua of the so-called hyoid bone of the adult. The 
 body of this bone represents the basal parts of the hyoid and 
 first branchial arch, the greater (posterior) cornua belonging to 
 the latter. 
 
 VI. LIMBS 
 
 The problem of the origin and morphological meaning of the 
 fins and limbs of Vertebrates is one which, in point of interest and 
 importance, is comparable to that relating to the head. During 
 the last thirty years it has been attacked vigorously both from the 
 embryological and the palaeontological sides, and has given rise to 
 so many speculations often of a very contradictory nature that 
 only the barest outline of some of the more important results 
 obtained can be given in the course of the present chapter. 
 
 The fins or limbs, which are distinguished from the axial organs 
 (head, neck, and body), as appendicular organs, serve mainly for 
 locomotion, and may be divided into two groups, the unpaired 
 and impaired (pectoral and pelvic). 
 
 A. Unpaired Fins. 
 
 The unpaired, or median fins, which are mainly characteristic 
 of Fishes, arise in the embryo as a ridge of the integument (epiblast 
 and mesoblast) extending along the median dorsal line from the 
 anterior part of the trunk backwards to the tail, around the apex 
 of which it is continued forwards for some distance along the 
 ventral side : thus a dorsal, caudal, and ventral portion can be 
 distinguished. In the course of further development, these 
 portions either remain continuous, or else certain parts undergo 
 reduction, so that the ridge only persists in certain regions, 
 where it forms independent dorsal, caudal, and ventral or anal fins 
 (Fig. 81, A, B) : in these regions muscles and skeletal parts be- 
 come developed. 1 
 
 These skeletal parts consist of supporting rays of two kinds. 
 In the base of the fin cartilaginous radii, usually segmented, are 
 
 1 The curious suctorial disc on the dorsal side of the head of the Teleostean 
 Remora (Echeneis), by means of which it attaches itself to foreign objects, arises 
 in the embryo from the anterior portion of the dorsal unpaired fin, and this is 
 indicated throughout life by the arrangement of the blood-vessels, nerves, and 
 skeletal parts. 
 
LIMBS 
 
 103 
 
 formed in all the Fishes proper and in Dipnoans ; these, which 
 may conveniently be distinguished as " pterygiophores" may unite 
 to form a Ixtsipterygium, and in bony Fishes they become ex- 
 tensively ossified : they frequently come into secondary con- 
 nection with the vertebral column (e.g., by means of the so-called 
 " interspinous bones" of Teleostei). The peripheral part of the 
 fin is supported by dermal rays, which may consist of numerous 
 delicate horny fibres (e.g., Marsipobranchs, Elasmobranchs, Cartilag- 
 inous Ganoids, Dipnoans), or of bony rods, entire or jointed, often 
 cleft at the base, and not preformed in cartilage (Teleosts, Bony 
 Ganoids). 
 
 Median fins are also present in the Amphibia, in which they 
 may persist throughout life (e.g., Perennibranchiata), or may only 
 
 BrF 
 
 FIG. 81. DIAGRAM SHOWING (A) THE UNDIFFERENTIATED CONDITION OF THE 
 PAIRED AND UNPAIRED FINS IN THE EMBRYO, AND (B) THE MANNER IN 
 WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS. 
 
 i>,"dorsal fin-fold ; S, S, lateral folds, which unite together at S 1 to form the 
 ventral fold ; RF, FF, dorsal fins ; SF, tail-fin ; A F, anal fin ; BrF, pectoral 
 fin ; BF, pelvic fin ; An, anus. 
 
 occur in the larval stage and occasionally also during the breeding 
 season (e.g. Newt). They have the form of a continuous in- 
 tegumentary fold extending round the tail and along the back 
 for a greater or less distance, but enclose no skeletal elements. 
 
 Amongst Reptiles, median fins were present in Ichthyosaurus, 
 and these are comparable to the dorsal fins occurring in the 
 Cetacea amongst Mammals : in both cases they must be looked 
 upon as structures acquired secondarily in connection with ao 
 aquatic existence. 
 
 B. Paired Fins or Limbs. 
 
 Embryological researches have shown that lateral fin-folds must 
 have existed in the ancestors of Vertebrates in addition to the 
 
104 
 
 COMPARATIVE ANATOMY 
 
 median fins, and these can still be recognised in young embryos 
 of Elasmobranchs (Fig. 81, A) and to a less extent in those of 
 Sturgeons, Teleosts, and Amphibians. They extended backwards 
 along the sides of the body from just behind the head, gradually 
 converging towards the anal region, where they became continuous 
 with the ventral part of the median fin-fold (Fig. 81, A), and thus 
 resemble the lateral or metapleural folds present in the adult 
 Amphioxus. As is usually the case in the median fins (p. 102), 
 certain parts of these lateral folds have undergone reduction, 
 only the anterior and posterior portions remaining to form two 
 paired (pectoral and pelvic) fins or limbs, which must therefore be 
 
 FIG. 81, A. TRANSVERSE SECTION THROUGH THE EMBRYO OF A SHARK (Pristiurus 
 melanoatomuft), 9 MM. LONG, SHOWING THE MODE OF ORIGIN OF THE PEC- 
 TORAL LIME-BUDS (ap.). 
 
 ch, notochord ; co, coelome ; m, myomeres, seen to be growing ventrally ; my, 
 
 spinal cord. 
 
 looked upon as the localised remains of a continuous lateral fin-fold 
 on either side of the body, and as being homodynamous (i.e., serially 
 homologous) structures. 
 
 Into these paired fins the myotomes extend, and cartilaginous 
 supports (pterygiophores) are formed from the mesoblast, as in the 
 case of the median fins. These radii appear first of all at the base 
 of the fin, gradually extending centrifugally into the latter, and 
 also, becoming fused, centripetally into the body-wall. An articula- 
 tion is then secondarily formed between the fused basal part of the 
 skeleton situated in the free portion of the limb (basipterygium) and 
 that which extends into the lateral body-wall and serves as a support 
 for the limb proper : this latter portion constitutes the lim~b-arcli 
 or girdle. The arch may remain comparatively small and not extend 
 
LIMBS 
 
 105 
 
 far dorsally ; but when the limb is destined to perform more im- 
 portant movements in locomotion, or to give a more definite support 
 to the body, the arch may extend upwards so as to come into 
 connection with the axial skeleton as well as meeting with 
 its fellow ventrally, thus forming an almost complete girdle 
 around the body. The limb skeleton may become ossified later. 
 
 FIG. 82. A, B, C. 
 
 DIAGRAM OF THREE SUCCESSIVE STAGES IN THE DEVELOP- 
 MENT OF THE PELVIC FIN OF A SHARK. 
 
 rd, primitive radii, which in A are beginning to fuse into a basal plate (bs}. In B 
 this fusion has taken place on both sides, and at * the proximal ends of the 
 two basals are approximating to form the arch. In C the process is com- 
 pleted, and at t an articulation has been formed between the arch and the 
 free portion of the fin. On the left side in C the radii are becoming second- 
 arily segmented, fo, obturator foramen ; cl, cloacal aperture. 
 
 In the case of Fishes, the pelvic fin as a rule remains at a simpler 
 and more embryonic stage than the pectoral. 
 
 The paired limbs are not connected with any particular body- 
 segments, but vary greatly in their relative positions and in the 
 number of nerves which supply them. 
 
 The essential part of this conception as to the origin of the paired limbs 
 is due to Thacher, Mivart, Balfour, Haswell, and Dohni. 1 Gegenbaur had 
 
 1 A somewhat similar idea was put forward by Goodsir as early as 1856. 
 
106 
 
 COMPARATIVE ANATOMY 
 
 previously put forward the view that the arches and fins correspond to meta- 
 morphosed gill-arches and rays : he supposed that one ray came to exceed 
 the others in size, and that the others then gradually became attached to it 
 instead of to the arch, the result being a biserial form of fin (" archiptery- 
 ") which is most nearly retained in Ceratodus (Fig. 101 and p. 124). 
 
 Pectoral Arch. 
 
 Fishes and Dipnoans. Paired fins and arches are wanting 
 in the Cyclostomi. In the Elasmobranchii and Holocephali the 
 pectoral arch consists of a comparatively simple cartilaginous bar 
 
 FIG. 83. PECTORAL ARCH AND FIN OF Heptanchu*. 
 
 SB, SB 1 , pectoral arch, with a nerve aperture at NL ; Pr, Ms, Mt, the three 
 basal elements of the fin pro-, meso-, and metapterygium ; Ra, cartilaginous 
 fin-rays ; a, b, the main fin-ray, lying in the axis of the metapterygium ; 
 t, single ray on the other side of the axis (indication of a biserial type) ; FS, 
 horny rays, cut through. 
 
 the two halves of which are united ventrally by cartilage or fibrous 
 tissue (Fig. 83), and in embryos of Ganoids and Teleosts it has 
 at first a similar structure. 
 
 Later, however, in both the last-named groups, a row of bony 
 structures arises in the perichondrium in this region ; so that a 
 secondary or bony pectoral arch may be distinguished from a 
 primary or cartilaginous one, the latter becoming less marked in 
 proportion to the development of the former (Fig. 84). 
 
 The free extremity, or fin, is always connected with the hinder 
 
PECTORAL ARCH 
 
 107 
 
 and outer circumference of the (primary) arch, convex articulations 
 being formed on the arch which fit into concave facets on the fin, 
 the point of attachment of 
 which may be taken as 
 separating the arch into an 
 upper dorsal and a lower 
 ventral section. The former, 
 which may exceptionally be 
 connected with the vertebral- 
 column (viz., Raiidas), cor- 
 responds to a scapula, and 
 the latter to a coracoid plus 
 procoracoid of the higher 
 Vertebrata. 1 
 
 In Teleosts and Bony 
 Ganoids the bony (secondary) 
 arch forms the principal 
 support of the fin in the 
 adult, the main element 
 being a large clavicle. The Co(Cl) 
 
 primitive relations are thus ^ 84 ._ LEFT PECTORAL AKCH 
 
 much altered. The arch 
 becomes secondarily con- 
 nected with the skull. (For 
 further details, compare 
 Fig. 84.) 
 
 Amphibia. In this 
 Class the pectoral arch 
 shows no direct connection 
 with that of Fishes, but is 
 similar in plan to that of 
 all the hio-her Vertebrates. 
 
 FIN OF 
 THE TROUT. (From the outer side.) 
 
 -D 1 , D 2 , chain of secondary bones of the 
 pectoral arch (clavicle and supra-clavicle), 
 which is connected with the skull by means 
 of the post-temporal (Cm) ; S and Co(Cl), 
 bony scapula and coracoid, which have be- 
 come developed in the cartilage (Kit) ; L 
 foramen in scapula ; J/ 1 , metapterygium ; 
 Ra, Ra, the second and third, and 4, the 
 fourth basal element of the fin ; Ra 1 , the 
 second cartilaginous row of radii ; US, 
 bony ray on the border of the fin which 
 is connected with the fourth basal element ; 
 F,S, bony fin -rays, shown cut away from 
 their attachments. 
 
 It always consists on either side of a 
 cartilaginous or bony dorsal plate (scapula), 
 which curves round the side of the body 
 and becomes continuous ventrally with 
 two processes an anterior (procoracoid) 
 and a posterior (coracoid) (Figs. 85 A and B). 
 The ventral part of the arch becomes con- 
 nected with the sternal apparatus (com- 
 pare Fig. 43). The humerus articulates 
 with a concave glenoid facet at the junc- 
 tion of the scapula and coracoid. The 
 two coracoid plates either overlap one 
 another in the mid-ventral line (Uro- 
 
 1 The pectoral arch of Dipnoans is intermediate in character between that of 
 Elasmobranchs and Ganoids. It shows so many special peculiarities as regards 
 form and position that it cannot be fully described here. 
 
 FIG. 85A. DIAGRAM OF THE 
 GROUND TYPE or PEC- 
 TORAL ARCH MET WITH 
 IN ALL VERTEBRATA FROM 
 AMPHIBIA UP TO MAM- 
 MALIA. 
 
 S, scapula ; Co, coracoid ; 
 Cl, procoracoid ; H, hu- 
 merus. 
 
108 
 
 COMPARATIVE ANATOMY 
 
 deles and certain Anura e.g., Bombinator, Fig. 43, C), or else 
 their free edges come into apposition and fuse together (other 
 Anura, e.g., Rana, see Fig. 43, D). In Anurans the procoracoids 
 have a more transverse position than in Urodeles, and come into 
 
 FIG. 85B. PECTORAL ARCH OF THE RIGHT SIDE OF Safamandra maattosa, 
 considerably magnified, and flattened out. 
 
 SS, supra-seapula ; S, scapula (ossified) ; Co, coracoid ; C/, procoracoid ; a, b^ 
 bony processes extending into the procoracoid and coracoid respectively ; 6',. 
 glenoid cavity, surrounded by a rim of cartilage (L). 
 
 connection with the coracoid in the mid-ventral line, thus giving 
 rise to a fenestra between the two. The whole arch is, moreover. 
 more strongly ossified, the procoracoid being covered by an invest- 
 ing bone the clavicle. 
 
 Reptilia. In Reptiles the ossification is still more 'marked. 
 
 The simplest condition of the shoulder-girdle is seen in Chelonians 
 
 (Fig. 86), in which its similarity to 
 that of Amphibians as well as to that 
 of Hatteria is at once seen : no clavicle 
 is developed. 
 
 In other Reptiles the same general 
 plan is retained with modifications. 
 Thus in Lizards (Fig. 44) the well- 
 developed clavicle is more indepen- 
 dent of the rest of the arch and 
 becomes ossified directly, forming a 
 
 S, scapula ; Co, coracoid; Co\ epi- delicate secondary bony lamella ex- 
 
 tending from the scapula to the apex. 
 
 r t i, pnistprml fmnarntiiQ "Rnt it 
 ot tn \ episternal appaiatUS. 
 
 must be remembered that the un- 
 differentiated cells of which it at 
 first consists are in direct continuity with those which form 
 the scapula, Unossified spaces are left in the coracoid, giving 
 
 Co 
 
 FIG. 86. PECTORAL ARCH OF A 
 CHELONIAN. (Ventral view. ) 
 
 coracoid ; Cl, procoracoid ; , 
 
 fibrous band between these two 
 elements ; Fe, fenestra between 
 them ; G, glenoid cavity. 
 
PELVIC ARCH 109 
 
 rise to fenestrae closed over by fibrous membrane. In Crocodiles 
 and Chameleons the clavicles are either wanting or rudimentary. 
 
 The preseiice of a pectoral arch in numerous footless Reptiles (certain 
 Skinks, Amphisbamiaiis) indicates that they formerly possessed extremities ; 
 rudiments of the latter may even be seen in the embryo though they 
 disappear entirely later on (Anguis fragilis). (For the peculiar pectoral 
 arch of the Stegocephala, see Fig. 46. ) 
 
 Birds. In Birds, the scapula consists of a thin and narrow 
 plate of bone often extending far backwards, the strong coracoid 
 being bent at a sharp angle with it in all Carinate Birds 
 (Fig. 41). The lower end of the latter is firmly articulated 
 in a groove on the anterior edge of the sternum. 
 
 In almost all Flying Birds the clavicle is well developed, and 
 'becomes united with its fellow to form bfurcula (comp. p. 63 and 
 Fig. 41). It is formed as a membrane bone investing a band of 
 cartilage present in the embryo in this region. 
 
 Amongst the Cursorial Birds, the Emeu and Cassowary possess 
 rudimentary clavicles : in the others they are wanting. They have 
 also undergone reduction in some Carinate Birds (e.g., certain 
 Parrots). 
 
 Mammals. In Monotremes only amongst Mammals does the 
 coracoid extend ventrally to reach the sternum (Fig. 48) ; in all 
 other members of this Class it characteristically becomes reduced, 
 and simply forms a prominent process on the scapula (coracoid 
 process), which becomes ossified from a separate centre. 1 
 
 Thus the scapula alone serves to support the extremity ; it 
 becomes at the same time greatly broadened out, and gives rise on 
 its outer side in connection with the highly differentiated muscles 
 of the limb to a strong ridge (spina scapulas), which extends 
 downwards to form the so-called acromion. The distal end of the 
 clavicle usually becomes connected with the acromion, its proximal 
 end articulating with the anterior edge of the sternum. 
 
 In those Mammals in which the fore-limbs are capable of very 
 varied and free movements, the clavicles are strongly developed. 
 In others, such as the Carnivora and Ungulata, they may be en- 
 tirely wanting or only rudimentary, and in the latter case their 
 relations to the scapula become altered. 
 
 Pelvic Arch. 
 
 Fishes. The first rudimentary indications of a pelvis are seen 
 in Cartilaginous Ganoids, amongst which, however, they present 
 considerable variations even in individuals of the same species. 
 They consist of two calcified or even ossified pelvic plates, which 
 
 1 According to Howes the coracoid process represents an epicoracoid (comp. 
 Fig. 48), the coracoid itself being only occasionally indicated by a small centre of 
 ossification on the glenoid margin of the scapula. 
 
110 
 
 COMPARATIVE ANATOMY 
 
 become segmented off from the basal cartilage (basi- or metaptery- 
 gium) of the free fin. In some cases even this segmentation does 
 not take place, and thus the pelvis remains undifferentiated. This 
 simple condition is also met with in the ancient forms Pleura- 
 canthus and Xenacanthus, and is essentially retained in Lepi- 
 dosteus, Amia, and the Teleostei (Fig. 87). 
 
 In Polypterus, which most nearly resembles the Devonian 
 Crossopterygii, the pelvis shows some advance on that of 
 Sturgeons. Owing, doubtless, to the necessity of a firmer connec- 
 tion of the fin with the body-wall, the two pelvic plates become 
 
 FIG. 87. DIAGRAMS ILLUSTRATING THE PHYLOGENY OF THE PELVIS. 
 
 A, Pleuracanthus the pelvis is here undifferentiated ft ; B, Scaphirhynchii* 
 cataphractus ; C, Polypterus bichir ; D, Necturus (Menobranchus). Bas 1 . 
 basipterygium ; Ap, its cartilaginous apophysis ; P, pelvis ; Bad, radii ; 
 Fo, obturator foramen. 
 
 united together in the mid-ventral line (Fig. 87, C) : but even here 
 the basipterygium may remain in continuity with the pelvic plate 
 on one or both sides. In spite, however, of the rudimentary 
 character of the pelvis of Polypterus, the essential form of that of 
 the Dipnoi and Amphibia is already sketched out. 
 
 The pelvis of the Elasmobranchii and Holocephali indicates that 
 they early branched off from the ancestral stock. Instead of 
 a small and narrow pelvic plate more or less elongated antero- 
 posteriorly, the pelvis forms a transverse bar of considerable 
 extent, developed in connection with the basipterygium (Fig. 82) 
 
PELVIC ARCH 
 
 111 
 
 it is perforated by nerves, and gives rise on either side to an iliac 
 process (most marked in Holocephali) extending into the lateral 
 walls of the body (Fig. 88). 
 
 In all the above 
 cases we may look pp 
 
 upon the pelvic plate 
 as essentially corres- 
 ponding, more or less 
 completely, with the 
 ischio-pvMs of higher 
 forms. 
 
 Dipnoi. The 
 
 small cartilaginous 
 pelvic plate (Fig. 89) 
 is provided with a 
 long and delicate an- 
 
 Rad;- 
 
 FIG. 88. DIAGRAM OF THE ELASMOBRANCH PELVIS. 
 
 , 
 
 S P' regi n f the ischi P ubic symphysis ; Fo\ 
 obturator foramen ; JBas, Pro, Rad, basiptery- 
 gium, propterygium, and radii of the fin. 
 
 (From the ventral side.) 
 
 terior median, a short BP, Pelvic plate (ischio-pubis) ; 7, iliac process ; 
 posterior median, and PP> prepubic process ; Cep, epipubic process , 
 
 two mi- of latpral 
 01 lateral 
 
 processes. Of the 
 
 latter the anterior 
 
 (prepubic processes) vary much in form and length, being much 
 
 longer in Protopterus than in Ceratodus, and each is embedded 
 
 in an intermuscular septum ; 
 with the posterior the skeleton 
 of the free fin is articulated by 
 means of an intermediate piece. 
 The anterior unpaired process 
 must be looked upon as an 
 epipubic process, corresponding 
 w ith that of Amphibians, Rep- 
 tiles, and Mammals (pp. 113, 
 115, 121). 
 
 Amphibia. Urodela. It 
 will be seen by a glance at 
 Fig. 87, D, that the ventral 
 portion of the pelvic arch of 
 Necturus is formed on the same 
 plan as the pelvic plate of the 
 Dipnoi and Crossopterygii, but, 
 
 ^ Tr , ^ as in all Urodela and Amniota, 
 
 .biG. 89. PELVIS or Protopterus. (From , f > > ,, , 
 
 the ventral side.) ^ 1S perforated by the obturator 
 
 . . nerve : this indicates a further 
 
 a, prepubic process, which may become i 4.^,^1 4. T -i J.T 
 
 forked at its distal end; b, process la ^ / S Jf"f WL 1 Llke the 
 to which the hinder extremity pelvis OI all Vertebrates, it has 
 
 (HE) is attached ; Gr, sharp ridge, a paired origin, and in Proteus 
 wffl&7t&25f; and Amphinma this is indi- 
 M 1 , M\ intermuscular septa. cated by the fact that its- 
 
FIG. 90. PELVIS OF (A) Proteus ; (B) Amphiuma ; (C) Cryptobranchus ; AND (D) 
 Salamandra macnlosa. (From the ventral side.) 
 
 JP, JP 1 , IP, ventral pelvic plate (ischio-pubis); ** (in .4), ossified region of the 
 ischium ; PP, prepubis ; ft (Cep), Ep, epipubis ; ** (in C and D) secondary 
 bifurcation of the epipubis ; z, outgrowth from this bifurcation ; t (in C), hypoi- 
 schiatic process, present in the Derotremata and Necturus ; Sy, symphysis, in 
 which region a strong tendinous area (SH) exists in Amphiuma, the pubic 
 regions only commg together in the middle line at * ; Fo, Fo 1 , obturator fora- 
 men ; Ac, acetabulum ; J, J*, I, I 1 , ilium ; Lalb, linea alba ; 3fy, intermuscular 
 septa ; Or, (Sy), muscular ridge on the ventral side of the ischio-pubis. 
 
PELVIC ARCH 
 
 113 
 
 FIG. 91. PELVIS OF VARIOUS AMPHIBIA. A, Xenopus (Dactylethra), from below ; 
 B, the same from the front ; C, Rana esculenta, from the right side ; D and 
 E, Salamandra atra ; F and G, Salamandra macutoxa ; H, Branchiosanrus ; 
 I, Discosaurus. D-I, from the ventral side. (Figs. H and I after Credner. ) 
 
 /, ilium ; Is, ischium ; P, pubis (P 1 in Rana, pubic end of ilium) ; IP, fused isehio- 
 pubic ossification ; PP, prepubis ; Cep, epipubic cartilage ; Fo l , obturator 
 foramen ; 7 1 (in Xenopus), the proximal end of the ilium, which is separated 
 from its fellow and from the pubis by a + -shaped zone of cartilage,!, * ; Ac, 
 acetabulum. 
 
114 
 
 COMPARATIVE ANATOMY 
 
 anterior epipubic process is paired throughout life (Fig. 90, A, B). 
 In the Derotremata and Myctodera, on the other hand, the 
 epipubis is unpaired from the first, owing probably to an abbrevia- 
 tion of development, its anterior end becoming bifurcated 
 secondarily (Fig. 90, C, D). 
 
 As in Fishes and Dipnoans, the two halves of the ischio-pubic 
 region tend to fuse together in the middle line to form an un- 
 paired pelvic plate, but all kinds of modifications 
 occur in this respect in adaptation to the move- 
 ments of the hind-limb in different forms ; and, 
 as in all cases the median zone of the plate 
 represents the line of least resistance, the lateral 
 halves may eventually become more or less dis- 
 tinct from one another. The effect of the action 
 of the muscles becomes, however, greater when 
 the pubic region is more distinctly marked off 
 from the ischium, and ossification takes place 
 in it (e.g., Salamandra atra and, more rarely, S. 
 maculata). Thus the typical triradiate arrange- 
 ment of the pelvis (ilium, iscJmim, and pulis}, 
 such as is further differentiated in certain Stego- 
 cephala (Discosaurus) and in Reptiles, as well 
 as in Xenopus, is already sketched out (Fig. 91). 
 
 An important difference between the pelvis 
 of Ganoids and Dipnoans and that of Amphibians 
 is seen in the marked development of the iliac 
 region in the latter group. The ilium, like the 
 scapula, extends upwards in the lateral walls of 
 the body (compare the iliac process of Elasmo- 
 branchs, Fig. 88), and in Proteus and Amphiuma, 
 owing to the reduction of the limbs in these 
 forms, does not reach the vertebral column (Fig. 
 90, A, B). In all other Amphibia, as in the 
 Amniota, it comes into connection with the 
 sacrum (p. 45), owing to the necessity for the 
 hind-limb to act as a support for the body in 
 terrestrial animals, and not merely as an organ 
 of propulsion, as in Fishes. 
 
 Anura. The pelvis of the Anura differs from 
 
 that of Urodela in the following characteristics. In correspondence 
 with their mode of progression, the ilium of each side becomes 
 extended so as to form a long rod (Figs. 91 C, 92) ; and the flat 
 pelvic plate, which in Urodeles lies in the plane of the abdominal 
 walls, becomes closely pressed together in the middle line and gives 
 rise to a well-marked ventral keel : it is not perforated by the 
 obturator nerve. The pubic region, moreover, though often calcified, 
 is independently ossified only in the case of Xenopus (Fig. 91, A,B). 
 
 Reptiles.' The chief characteristics of the Reptilian pelvis as 
 
 FIG. 92. PELVIC 
 ARCH OF FROG 
 
 { Ra na esculent a}. 
 From below. 
 
 J, J 1 , ilium ; Is, is- 
 chium ; P, carti- 
 laginous pubic 
 region ; Or, the 
 median ventral 
 ischio-pubic 
 crest ; G, aceta- 
 bulum ; Oc, uro- 
 style ; Pt, trans- 
 verse process of 
 sacral vertebra. 
 
PELVIC ARCH 
 
 A 
 
 FIG. 93. PELVIC ARCH OF VARIOUS REPTILES. (From the ventral side). A, 
 Jalceohctitena, after Credner ; A'-C, Plesiosaurus : A 1 , from a restoration in 
 the College of Surgeons; B, from Huxley's Anatomy of Vertebrated 
 Hattwia ^^ D ' Arcy Thom P son ; V, Labyrinthodon riitimeyeri ; E, 
 
 P, pubis ; PP prepubis ; Cep, epipubic cartilage ; Fo\ obturator foramen ; Is, 
 iscnmm ; /, ilium ; f,f, isohio-pubic foramina ; *, hypoischiatic process, which 
 becomes segmented off from the pelvis in other Reptiles. 
 
 I 2 
 
116 
 
 COMPARATIVE ANATOMY 
 
 A 
 
 FIG. 94. PELVIC ARCH OF VARIOUS CHELONIANS. (From the ventral side.) A, 
 Macrochelys (after G. Baur) ; B, median pelvic cartilage of Chelys fimbriata ; 
 C, the same of Emydura ; D, Sphargis coriacea (after Hoffmann) ; E, Testudo ; 
 F, Chelone. 
 
 Cep, epipubic cartilage ; Hpls, hypoischiatic process ; P, pubis ; PP, prepubis ; 
 Is, ischium ; Fopi, ischio-pubic foramen. 
 
 compared with that of Amphibians consist in : (1) a much more 
 marked differentiation of the pubis, which is more distinctly 
 separated from the ischium by an ischio-pubic foramen ; (2) the 
 
PELVIC ARCH 
 
 117 
 
 greater development of the ilium, which is sometimes broadened 
 out at its vertebral end ; and (3) the more intense and solid ossifica- 
 tion of the arch as a whole. 
 
 Points of connection with the pelvis of Amphibians are seen in 
 Palseohatteria, the Plesiosauria, Hatteria, Telerpeton, and the 
 Chelonia (comp. Figs. 93 and 94), while the pelvis of the Ichthyo- 
 sauria approaches that of the Lacertilia. In the latter, and still 
 more in the Crocodilia and Dinosauria, the pelvic arch is much 
 
 FIG. 95. A, LONGITUDINAL HORIZONTAL SECTION THROUGH THE VENTRAL PART 
 OF THE PELVIS OF AN EMBRYO OF Lacerta agilis, 32 MM. IN LENGTH. B, 
 PELVIS OF Lacerta vivipara. (From the ventral side. ) 
 
 Ep, epidermis ; P, pubis ; PP, prepubis ; Is, ischium, forming a symphysis at 
 Sis ; Hpls, hypoischium, which becomes segmented off from the hinder ends 
 of the ischia in the embryo as a paired structure ; f, dense mass of embryonic 
 tissue ; /, ilium, with its small preacetabular process ft, which is much 
 more strongly developed in Crocodiles, Dinosaurians and Birds ; Ac, aceta- 
 bulum, in which the three pelvic bones unite together so that the sutures 
 between them become obliterated ; Fo l , obturator foramen ; Cep, epipubis, 
 composed of calcified cartilage ; Lg, fibrous ligament. 
 
 more highly differentiated ; while in Snakes, on the other hand, it, 
 like the pectoral arch, is entirely wanting. 
 
 In Hatteria (Fig. 93 E) there is a marked epipubis and a hypo- 
 ischiatic process continuous with the epipubic cartilage, and the 
 prepubic processes are strongly developed. The obturator and 
 ischiopubic foramina are distinct from one another, and not united 
 into one, as in Chelonia. (For the various modifications seen in 
 the pelvis of the latter Order, more particularly as regards the 
 relative development of the epipubic and prepubic processes and 
 the relations of the ischium and pubis, compare Fig. 94.) 
 
118 
 
 COMPARATIVE ANATOMY 
 
 The pelvis of the typical Lacertilia (Fig. 95 B) is characterised by 
 a lightness of build. The rod-like pubis and ischium are separated 
 from one another by large ischiopubic foramina, and between them 
 in the middle line is a longitudinal fibre-cartilaginous ligament, 
 continuous anteriorly with the plug-like epipubic cartilage and 
 posteriorly with the hypoischium. This tract represents the 
 
 FIG. 96. PELVIS OF A YOUNG Alligator Indus. (A, ventral, and B, side view.) 
 
 If, ilium ; Is, ischium ; P, pubis ; Sy, symphysis of ischium ; F, ischio-pubic 
 foramen ; JB, fibrous band between the symphyses pubis and ischii ; f, pars 
 acetabularis, which is interposed between the process a of the ilium and the 
 pubis ; 6, foramen in the acetabulum, bounded posteriorly by the two pro- 
 cesses, a and b, of the ilium and ischium respectively ; *, indication of a 
 forward growth of the ilium, such as is met with in Dinosaurians and Birds ; 
 G, acetabulum ; 7, II, first and second sacral vertebrae ; M, fibrous mem- 
 brane extending between the ant rior margin of the pubis and the last pair 
 of "abdominal ribs" (BR.) 
 
 remnant of the median ends of the pubis and ischium which 
 are present in the embryo (Fig. 95 A) ; and thus in this, as in 
 certain other respects, the pelvis of the Lacertilia may be said to 
 pass through a Hatteria-like stage in the course of development. 
 The epipubis and hypoischium arise as paired rudiments. The 
 ilium in some cases is almost vertical in position : in others it is 
 
PELVIC ARCH 
 
 119 
 
 more oblique, sloping upwards and backwards from the aceta- 
 bulum. 
 
 The pelvis of Crocodiles exhibits special characteristics and is 
 of particular interest, as in some points it resembles that of 
 certain extinct forms. The pubes, which have at first a trans- 
 verse position, become later directed forwards much more 
 markedly than in Chelonians and Lizards, and thus the ischio- 
 pubic foramina (in which the. obturator foramina are included) are 
 very wide, and are separated from one another by a fibrous cord 
 (Fig. 96). A symphysis, both of the pubis and ischium, is formed, 
 but the former is not present in the adult. The acetabulum is 
 perforated, and the pubis is separated from it by a cartilaginous 
 pars acetdbularis, not represented in lower Vertebrates, formed 
 from the acetabular process of the ilium. The epipubis is 
 possibly represented by a cartilaginous apophysis at the anterior 
 (distal) end of the pubis, but it never becomes separately 
 differentiated. 
 
 The ilium becomes greatly broadened out in the antero- 
 posterior direction dorsally, where it is attached to the sacrum ; and 
 this is of special interest as a similar extension of the ilium 
 occurs still more markedly in Dinosaurians and Birds (Fig. 97). 
 
 Birds. The pelvis of Birds is chiefly characterised by the 
 relatively large development of the iliac region and by the position 
 of the delicate pubis, which in the course of development becomes 
 
 FIG. 97. PELVIS OF Apteryx australis. Lateral view. (After Marsh.) 
 
 r7, ilium ; is, ischium ; p, spinous process from the pars acetabularis ; p 1 , pubis ; 
 
 a, acetabulurn. 
 
 directed backwards, parallel to the ischium and post-acetabular 
 process of the ilium, and is often united with the ischium 
 
120 
 
 COMPARATIVE ANATOMY 
 
 (Carinatas). The preacetabular portion of the ilium extends for- 
 ward for a considerable distance, and a number of vertebrae 
 belonging to other than the true sacral region become secondarily 
 connected with the ilium (see p. 48). The acetabulum is per- 
 forated, and the pars acetabularis (p. 119) forms a spinous process. 
 The elements of the pelvis usually become anchylosed together. 
 The pubis meets its fellow in the middle line only in Struthio, 
 and the ischium only in Rhea. 
 
 Mammals. The elements of the pelvis here remain separated 
 for a long time by cartilage, but later they become fused together. 
 The pubis always takes less part in the formation of the aceta- 
 bulum than do the other two bones, and may be more or less 
 entirely shut out from it by an ossification of the pars acetabularis, 
 which subsequently unites with either the ilium, ischium, or 
 pubis (Figs. 98 and 99). This acetdbular lone is especially well 
 developed in the Mole, in which it shuts the ilium, as well as the 
 pubis, out of the acetabulum : the latter is perforated in Mono- 
 tremes. The angle between the axes of the ilium and sacrum is 
 large in Orriithorhynchus, and more acute in other Mammals. 
 
 The original type with both pubic and ischiatic symphyses is 
 seen in Monotremes, Marsupials (Fig. 100), many Rodents, In- 
 sectivores and Ungulates. In many other Insectivores, in Carnivores, 
 and more particularly in the Primates, the ischia no longer meet 
 below. The greatest amount of variety in the form of the pelvis 
 
 FIG. 98. EXTERNAL VIEW OF THE 
 RIGHT HALF OF THE HUMAN 
 PELVIS. (From the outer side.) 
 
 The three bones ilium (//), ischium 
 (Is}, and pubis (P) are shown dis- 
 tinct from one another in the 
 acetabulum. Fo, obturator for- 
 amen. 
 
 FIG. 99. DIAGRAM SHOWING THE 
 RELATIONS OF THE PARS ACETA- 
 BULARIS (in Viverra civetta). 
 
 J, ilium ; Js, ischium ; P, pubis ; 
 A, acetabular bone ; Ac, aceta- 
 bulum. 
 
 in any one order is seen in Insectivores, in some of which (e.g., 
 Mole), as well as in most Bats, there is no symphysis pubis. The 
 obturator foramen is always surrounded by bone. 
 
PELVIC ARCH 
 
 121 
 
 In Whales, in which hind-limbs are wanting, paired rudiments of the 
 ischio-pubic region of the pelvis are present. They are unconnected with 
 one another and with the vertebral column. 
 
 In Monotremes and Marsupials of both sexes, two strong so- 
 called "marsupial bones" (Fig. 100) arise from the anterior 
 border of the pubes, right and left of the middle line, and extend 
 forward in a straight or oblique direction embedded in the 
 
 Tul.H.p , 
 
 + -* Ti 
 
 FIG. 100. PELVIS OF A, Echidna hystrix (ADULT), AND B, Didelphys azarce 
 (F(ETUS, 5'5 CM IN LENGTH). (From the ventral side.) 
 
 Ep, epipubis ("marsupial bone") ; P, pubis ; Sy, ischiopubic symphysis ; Js, 
 ischium ; J, ilium ; Fobt, obturator foramen ; Tub.il. p, ilio-pectineal tubercle ; 
 Lg and Lgt, ligament between the pubis and epipubis ; **, cartilaginous 
 apophysis at the anterior end of the epipubis. 
 
 In Fig. A, t*, t, tt, ilio- and ischio-pubic sutures ; Z, process on the anterior 
 border of the pubis ; GH, articulation between the pubis and epipubis ; Tb, 
 cartilaginous tuber ischii. 
 
 In Fig. B, b, 6 1 , cartilaginous base of the epipubis, continuous with the inter- 
 pubic cartilage at t ; *, *t, ischio-pubic and ischio-iliac suture. 
 
 body .-walls. They form an integral part 'of the pelvis, and in the 
 embryo are seen to be in direct connection with its cartilaginous 
 symphysis ; but later on articulations are formed between them 
 and the pubes. There can be no doubt that these structures are 
 the homologues of the epipubis of lower Vertebrates, which has 
 been retained in non-placental Mammals in order to serve as a sup- 
 port for the abdominal walls in connection with the marsupial 
 pouch (p. 28). 
 
122 
 
 COMPARATIVE ANATOMY 
 
 -a, 
 
 FREE LIMBS. 
 Fishes and Dipnoans. 
 
 In the following description the pelvic fin will be considered 
 before the pectoral, as it usually retains a simpler and more 
 primitive form. 
 
 ,$ Elasmobrancliii and Holocephali. The cartilaginous skeleton of 
 the fins is the most richly segmented in these Fishes. There 
 
 are usually two main elements (basalia) 
 in the pelvic fin which articulate with 
 the arch and with which a variable 
 number of segmented rays (radii) are 
 connected, the latter passing towards 
 the periphery of the fin (Fig. 88). Both 
 the larger, posterior main element (basi- 
 or metapterygiiim), and the smaller, 
 inconstant propterygium must, as al- 
 ready stated (p. 105), be looked upon 
 as originating phylogenetically, at any 
 rate by a fusion of the proximal ends 
 of the primary cartilaginous rays of the 
 fin; and the form and relations of 
 these main elements vary according to 
 the degree in which such a fusion 
 has taken place. 1 This is also true 
 as regards the pectoral fin, in which an 
 additional basal piece, or mesoptery- 
 gium, is usually present between the 
 pro- and metapterygia, and, like these, 
 articulates with a special convexity on 
 the pectoral arch (Fig. 83) : there may 
 even be four basalia. These compli- 
 cations arise in connection with the 
 greater importance of the pectoral than 
 the pelvic fin as an organ of locomotion. 
 The distal portions of both fins are 
 supported by horny fibres (p. 103). 
 With the exception of one (Fig. 83, t) 
 or at most of very few all the rays are 
 situated on the same side of the basalia 
 (uniserial type). 
 
 Dipnoi. The cartilaginous pectoral and pelvic fins are here 
 also essentially similar to one another, the latter being 
 rather the simpler of the two. From a segmented main-ray or 
 
 1 In male Elasmobranchii and Holocephali a number of pieces of cartilage are 
 connected with the distal end of the metapterygium of the pelvic fin as a support 
 for the copulatory organs or claspers : these may become more or less calcified. 
 
 FIG. 101. PECTORAL FIN OF 
 
 Ceratodua fosteri. 
 
 a, b, the two first segments of 
 the main axial ray ; t, t, 
 lateral rays ; FS, horny 
 rays, shown only on one 
 side. 
 
LIMBS 
 
 123 
 
 axis a number of segmented secondary rays arise on either side in 
 Ceratodus : these are not, however, strictly symmetrical (Fig. 101). 
 Beyond them horny rays are present, as in Elasmobranchs. A 
 proximal (basal) segment of the axis, which bears no rays, articu- 
 lates with the arch. In Protoptertis and Lepidosiren the fins, with 
 their skeleton, have undergone a marked reduction, so that little 
 more than the segmented axis remains. 
 
 Thus the fins of Dipnoans differ from those of Elasmobranchs 
 (as well as of Ganoids and Teleosts) in being formed on a Mserial 
 type. 
 
 Ganoidei. The skeleton of the fin is much simpler and the 
 
 A 
 
 FIG. 102. LEFT PECTORAL FIN OF A, Polyodon (Spatularia), AND B, Amia. 
 
 I- IV, cartilaginous radii connected with the arch (S) ; a-g, radii which do not 
 reach the arch and are connected with the most posterior ray (IV in A, III 
 in B) : KS, bony rays. 
 
 primary rays much fewer in number in Ganoids than in Elasmo- 
 branchs. 
 
 In the pelvic fin of cartilaginous Ganoids more or fewer of the 
 radii unite together proximally to form a basale, which is perforated 
 by nerves, and from which a very primitive pelvic plate becomes 
 differentiated (p. 109, Fig. 87. B). It is important to bear in mind 
 that the distinction between an axis and secondary rays cannot 
 here, therefore, be strictly recognised, and the fin is thus more 
 primitive than in Elasmobranchs. 
 
 The primitive relations have to a certain extent disappeared in 
 the pectoral fin of cartilaginous Ganoids, which, however, consists 
 of a varied number of rays. Of these, four reach the arch in 
 Polyodon (Fig. 102, A) and five in Acipenser. 
 
 Fn the pectoral fin of Amia (Fig. 102, B) two large converging 
 marginal rays articulate with the shoulder-girdle, and only one 
 
124 
 
 COMPARATIVE ANATOMY 
 
 intermediate ray reaches the arch : this condition maybe compared 
 with that seen in the highly-developed pectoral fin of Polypterus 
 (comp. Fig. 103). 
 
 The form of the pelvic fin in bony Ganoids may be easily 
 derived from that seen in the cartilaginous representatives of this 
 Order, but the number of radii is greatly reduced (Fig. 87). The 
 
 rays supporting the distal part of 
 both pairs of fins are bony (comp. 
 p. 103). 
 
 Teleostei. A still further reduc- 
 tion has taken place in the primitive 
 skeleton of the paired fins in Tele- 
 osts, there being at most only a few 
 radials articulating with the arch 
 (Fig. 84), and even these (especially 
 in the case of the pelvic fin) may be 
 wanting. The main part of each fin 
 is supported by bony rays, as in 
 osseous Ganoids. The skeleton of 
 the fins of Siluroids, Cyprinoids, 
 and GymnotidaB comes nearest to 
 that of Ganoids. 
 
 FIG. 103. PECTORAL FIN OF 
 Polypterus. 
 
 Pr, Ms, Ml, pro-, meso-, and meta- 
 pterygium, the first and last 
 
 Phylogeny of the Ichthyopterygium. 
 
 Two essentially different views 
 exist as to the primitive form of 
 fin-skeleton in Fishes. As already 
 mentioned on p. 106, Gegenbaur 
 postulates a biserial fin as the primi- 
 tive type (archipterygium), which 
 is most clearly retained in Cera- 
 todus. He supposes that the 
 uniserial form has been derived 
 from this by a reduction of the rays 
 on one side and a further develop- 
 ment of those on the other. The 
 axial ray of the biserial fin would 
 thus correspond to the basi- or 
 metapterygium, while the pro- and 
 
 mesopterygia of the uniserial fin would answer to special develop- 
 ments of the proximal ends of certain of the rays on one side of it. 
 The other view, which seems a far more probable one, is that 
 the uniserial type is the more primitive, and that this type is most 
 nearly retained in Elasmobranchs, which are as ancient a group as 
 the Dipnoans and which have not passed through a Dipnoan stage 
 in the course of their phylogenetic development. 
 
 Fig. 82 represents the mode of origin of the Elasmobranch fin 
 
 reach the arch ; OSS, centre 
 of ossification in MS ; * part of 
 the mesopterygium which ex- 
 tends between the distal end of 
 the propterygium and the first 
 row of radii ; Nl, nerve for- 
 amina in the mesopterygium ; 
 Ra, Ra l t radii ; FS, bony der- 
 mal rays. 
 
LIMBS 
 
 125 
 
 in accordance with this view, which is further supported by many 
 of the facts stated above and by numerous others relating to the 
 structure and development of the fins in Fishes, as well as by a 
 study of such fossil forms as the Palaeozoic Cladoselache. 
 
 GENERAL CONSIDERATIONS ON THE LIMBS OF THE 
 HIGHER VERTEBRATA. 
 
 It thus appears possible to derive the skeleton of the fin of all 
 the orders of Fishes from a single ground-type, but to trace the con- 
 nection of the latter with the extremities of Amphibia and Amniota 
 is a far more difficult task. Between these two types of limb there 
 seems to be a wide gap, in consequence of the different conditions 
 of life existing between aquatic and terrestrial Vertebrates : we do 
 not know how the limb of an air- 
 breathing Vertebrate (cheiroplery- 
 yiuvi), adapted for progression upon 
 Jand, has been derived from the fin 
 (ichthyopterygium), only fitted for use 
 in the water. 
 
 Palaeontology furnishes no answer 
 to this question ; we know of no 
 fossil intermediate forms of limb, 
 and the various hypotheses which 
 have been put forward on the sub- 
 ject cannot be discussed here. We 
 may suppose that when the primitive 
 Amphibian first began to take on a 
 terrestrial mode of life, its fin, which 
 is practically a single-jointed lever, 
 amply sufficient for the movement of 
 the body in a fluid medium, became 
 gradually transformed into a many- 
 jointed system of levers. 
 
 In other words, as the function of the limb was no longer 
 simply to propel the body, but also to lift it up from the 
 ground, the firmly-connected elements of the skeleton of the 
 tin gradually became loosened from, and placed at an angle 
 to, one another (knee, elbow), definite articulations being 
 formed between them in a .proximo-distal direction. Moreover, 
 the extremity must have changed its position with regard to the 
 body, so that, instead of projecting horizontally outwards, it became 
 bent downwards, and thus the angle between it and the median 
 plane of the trunk was gradually reduced, until in Mammals even- 
 tually, the longitudinal axis of the limb, when at rest, oame to lie 
 parallel with the median plane of the body. In the higher types 
 this is more particularly the case as regards .the posterior ex- 
 
 FIG. 104. DIAGRAMMATIC FIGURES 
 
 TO SHOW THE RELATIONS OF THE 
 
 ANTERIOR FREE EXTREMITY TO 
 THE TRUNK IN FISHES (A), AND 
 THE HIGHER VERTEBRATES (B). 
 
 S, pectoral arch ; Mt, metaptery- 
 gium ; Rd, radialia in A, radius 
 in B ; Ul, ulna ; proximally to 
 Ul and Rd is the humerus. 
 
126 
 
 COMPARATIVE ANATOMY 
 
 tremities, the anterior undergoing the most varied adaptative 
 modifications, and giving rise to tactile, prehensile, or flying organs 
 or, as in aquatic Mammals, becoming once more converted into 
 
 rowing organs. The limbs of all 
 the higher Vertebrata may, how- 
 ever, also be reduced to a single 
 ground-type. 
 
 The fore- and hind-limbs show a 
 great similarity as regards the form 
 and position of their various parts. 
 A division into four principal sec- 
 tions can always be recognised : 
 in. the case of the fore-limb these 
 are spoken of as upper arm (brack- 
 ium), fore-arm (antibrachium), wrist 
 (carpus), and hand (manus) ; and 
 in the hind-limb as. thigh (femur), 
 shank (cms), ankle (tarsus), and 
 foot (pes) (Figs. 105, 106). The 
 bone of the upper arm (humerus) 
 and of the thigh (femur} is always 
 unpaired, but two bones are present 
 in the fore-arm and shank. The 
 former are called radius and ulna, 
 and the latter tibia and fibula. The 
 hand and foot are also respec- 
 tively divisible into two sections, a 
 proximal metacarpus and metatarsus, 
 and a distal series of. phalanges, 
 which form the skeleton of the 
 fingers and toes (digits). 
 
 Both manus and pes are made 
 up of several series of cylindrical 
 bones. There are never more than 
 five complete series, which except 
 
 FIG. 105. SKELETON OF THE RIGHT 
 FORE-ARM, CARPUS, AND HAND 
 OF Salamandra maculosa. (From 
 above. ) 
 
 JR, radius ; 7", ulna ; r, radiale ; i, 
 u, intermedio-ulnare ; c, cen- 
 trale ; 1 to 4, first to fourth 
 carpalia (according to Emery, 
 
 l corresponds to the carpal of as regards number present essen- 
 the prepollex and 2 to the tially similar primary relations 
 common carpal of digits / and , i i , i i i '-\r , i 
 
 //) ; Me, Me, metacarpals ; throughout the higher Vertebrates. 
 Ph, phalanges ; 7 to IV, first The skeleton of the carpus and 
 to fourth fingers. tarsus, each of which always consists 
 
 of a series of small cartilages or 
 
 bones, shows much variation; but the following may be taken 
 as a ground-type (Figs. 105 and 106). Round a centrale, which 
 may be double, is arranged a series of other elements, of 
 which three are proximal, and a varying number (four to six) 
 distal. The proximal, in correspondence with their relations 
 to the bones of the fore-arm and shank respectively, are spoken of 
 as radiale or tibiale, ulnare or fibulare, and intermedium ; while the 
 
LIMBS 
 
 127 
 
 distal are called carpalia or tarsalia (in the narrower sense). They 
 are counted beginning from the pre-axial (radial or tibial) side of 
 the limb. 
 
 Amphibia. The anterior and posterior extremities of Urodela 
 are formed essentially on the ground-plan described above. There 
 are usually five digits in the hind-limb, and always four in the 
 fore-limb. In the Anura the radius and ulna become fused together, 
 and a separate intermedium is wanting ; the proximal row of the 
 tarsus, moreover, consists of only two cylindrical bones, one of which 
 (astragalus} corresponds to a tibiale, and the other (calcaneum) 
 to a fibulare. 1 
 
 In the distal row of the carpus four separate elements are 
 formed, but this number may become reduced owing to secondary 
 fusions ; in rare cases a fifth carpal may 
 also be present. Very different views 
 exist as regards the homologies of the 
 individual carpals of Anurans. In the 
 distal row of the tarsus, tarsalia // and 
 /// are the most constant elements, 
 but even these may undergo fusion ; 
 tarsalia IV and F are generally repre- 
 sented by a ligament ; and tarsale / 
 usually does not long remain distinct. 
 
 In Anura the metatarsals and 
 phalanges, between which the web of 
 the foot is stretched, are very long and 
 slender. The femur, as well as the 
 bones of the shank, which are fused 
 together, are also exceedingly long, in 
 correspondence with the mode of pro- 
 gression of these animals. The skeleton 
 of the extremities is more strongly 
 ossified in Anurans than in Urodeles, 
 in which many of the elements remain 
 cartilaginous. 
 
 Traces of an extra toe (prehalhwf) 
 occur on the tibial side of the tarsus, and in both Urodeles and 
 Anurans indications of an additional pre-axial digit in the manus 
 are occasionally met with. The number of phalanges on the 
 individual digits varies in different Amphibians. 
 
 Rudiments of the extremities can be recognised externally in embryos of 
 the limbless Gymiiophiona. 
 
 Reptiles. Chelonians and Lizards (and more especially Hat- 
 
 Fio. 106. SKELETON OF 
 
 SHANK, TARSUS, AND^FOOT 
 OF Spelerpes fuscus. 
 
 ib, tibia ; fb, fibula ; t, tibiale ; 
 i, intermedium ; f, fibu- 
 lare ; c, centrale ; 1-5, 
 tarsalia; i-v, digits. 
 
 1 It is possible that the tibiale and fibulare also include the representatives of 
 other elements. 
 
128 
 
 COMPARATIVE ANATOMY 
 
 teria) l closely resemble Urodeles in the structure of the carpus, 
 although the exact homologies of all the different elements can- 
 not yet be stated with certainty. Five digits are always present 
 in both manus and pes, and in Chelonians traces also of the former 
 possession of an extra finger both on the radial and ulnar side 
 
 ("pisiform") are t be seen ( Fi g s - 10 ^ 108 > and 109 )- Tne ^ia 
 and fibula always remain separate. 
 
 In Crocodiles, which, like Anurans, possess no trace of an 
 intermedium, the proximal row of the carpus consists of two hour- 
 glass-shaped bones a larger radiale, and a smaller ulnare (Fig. 
 110). A rudiment of a sixth ray is present on the outer side of 
 
 A 
 
 FiG. 107. CARPUS OF A, Hatteria (Sphenodon) punztatri, AND B, Emydura 
 kre/tii. (After Baur. ) 
 
 R, radius ; U, ulna ; r, radiale ; u, ulnare ; i, intermedium ; c 1 , radial centrale ; 
 -, ulnar centrale; 1-5, carpalia ; p, ulnar sesamoid (pisiform); I-V, the 
 metacarpals. 
 
 the latter. The centrale, as in Anura, comes to be situated in the 
 distal row, which is much less developed than the proximal. 
 
 In all Reptiles the tarsus undergoes a marked reduction, 
 especially in its proximal portion, and gradually leads to the 
 type seen in Birds. Thus in Chelonians and Lizards the proximal 
 tarsals all run together into a single mass which corresponds to the 
 tibiale, intermedium, fibulare, and centrale, and the last mentioned 
 element can no longer be recognised in Lizards, even in the embryo. 
 Traces of an extra radial ray are present. 
 
 In the distal row three or four (five in Palseohatteria) separate 
 tarsals are developed, but these may unite partly with one another 
 
 1 In Hatteria and Chelydra serpentina amongst existing Reptiles, a double 
 centrale is present in the carpus, and traces of a double condition of this element 
 are seen in certain other Chelonians. 
 
LIMBS 
 
 129 
 
 (Chelonians), and partly with the corresponding metatarsals 
 (Lizards) ; thus there is an increasing tendency for the move- 
 ment of the foot to take place by means of an intertarsal articula- 
 tion, as in Birds. 
 
 In. Crocodiles there are two bones in the proximal row of the 
 tarsus, one of which corresponds to a tibiale, intermedium, and 
 centrale, the other to a fibulare. The former is spoken of as the 
 astragalus, the latter as the calcaneum, and on it a definite heel 
 (calcaneal process) is seen for the first time in the animal series. 
 
 T JT 111 w 
 
 FIG/ 108. RIGHT CARPUS OF Emys 
 europcea. (From above. ) 
 
 R, Radius ; U, ulna ; r.c, fused radiale 
 and centrale (or centrale 1 and 2, 
 Baur) ; i, intermedium ; u, ulnare ; 
 1-5, the carpalia, of which 4 and 
 5 have become fused together ; t 
 (radiale, Baur) and *, elements 
 on the radial and ulnar side res- 
 pectively, indications of additional 
 radial and ulnar (pisiform) rays ; 
 I-V, the metacarpals. 
 
 FIG. 109. LEFT CARPUS OF 
 Lacerta ayilis. (From 
 above.) 
 
 ./?, radius ; U, ulna ; u, ul- 
 nare ; i, intermedium ; r, 
 radiale, formed by the 
 fusion of two elements, 
 one of which corresponds 
 to a prepollex ; c, cent- 
 rale ; 1-5 ; carpalia ; f> 
 ulna sesamoid (pisiform); 
 /- V, the metacarpals. 
 
 The distal row consists originally of four small cartilages, but these 
 later undergo a partial fusion. 
 
 The number of phalanges on the fourth and fifth digits in the manus is 
 greater in the embryos of Crocodiles than in the adult. This indicates that 
 the Crocodilia have been derived from forms possessing a fin-like fore-limb. 
 
 In Ichthyosaurus and Plesiosaurus the limbs are modified to form paddles, 
 the digits consisting of numerous phalanges, and additional rays being present 
 in the former genus. In Pterodactylus and Rhamphorhynchus the fourth finger 
 was produced into a long jointed rod, which supported a wing-like expansion 
 of the integument. 
 
 Amongst the Lacertilia, various degrees of reduction of the extremities 
 may occur, and in certain Snakes (e.g., Python) traces of the hind-limbs exist. 
 
 Birds. The fore-limb of Birds is considerably modified by 
 adaptation for flight. The manus loses its primitive character and 
 undergoes reduction, while the brachium and antibrachium, as 
 
130 
 
 COMPARATIVE ANATOMY 
 
 7 
 
 well as the entire pectoral arch and sternum, are extraordinarily 
 developed. In the Ratit^, however, the wing has undergone 
 regressive changes in connection with the habits of these Birds. 
 
 Of the six or seven carpals which may be present in the embryo, 
 the three distal become fused with the corresponding metacarpals, 
 thus forming a carpometacarpus (Figs. Ill, 112 A), and in the 
 adult only the two proximal remain separate as a radiale and an 
 ulnare. The three metacarpals themselves 
 become united together proximally, and the 
 second and third distally : they only bear a 
 very limited number of phalanges" at their 
 free ends. 
 
 Claws were present on the terminal phalanges 
 of all three digits in Archaeopteryx. In certain 
 recent Birds the first digit bears a claw, and more 
 rarely the second and even the third also. 
 
 The tarsus is still more reduced in Birds 
 than in Reptiles, and consists in the embryo 
 of three elements, two small proximal and a 
 broader distal. The former (tibiale and 
 fibulare) unite later with the distal end of 
 the tibia, thus forming a tibiotarsus, while 
 the latter, which corresponds to tarsalia / to 
 V, becomes included in the base of the 
 metatarsus. Thus the foot of adult Birds 
 radiale (including, no longer possesses any distinct tarsal ele- 
 according to Em- men ts, though, as in Chelonians and Lizards, 
 
 ery, a carpal of the xl ~ i , i 
 
 prepollex); u, ul- tne * 00 "k really moves by an intertarsal articu- 
 
 nare; C, centrale ; lation. Of the original five metatarsals, the 
 
 l to 5, the five car- fifth soon disappears, while the second, third, 
 palia, as yet unossi- 1^-11 -1-^1 
 
 fied, of which l and an d fourth become united with one another 
 
 2, as well as 3, 4, and with the distal element of the tarsus 
 and 5 have become to f orm a single bone, the tarsometatarsus 
 
 fused together ; t, /-p.- m -, -f o -r>\ mi n 
 
 pisiform T / to V, (Figs. Ill, 112 B). The first metatarsal 
 
 the metacarpals. remains to a greater or less extent inde- 
 pendent. 
 
 The number of toes varies between two (Struthio) and four ; 
 that of the phalanges is normally 2, 3, 4, 5, reckoning from the 
 first to the fourth digit. The tibia, even from the first, greatly 
 exceeds the fibula in size, and the two bones become fused to- 
 gether distally. 
 
 In both Jimbs the bones are usually pneumatic. (See under 
 Air-sacs.) 
 
 Mammals. In Mammals the anterior extremity either re- 
 mains in the condition of a simple organ of locomotion, serving for 
 progression on land ; or it may become modified in adaption to an 
 
 ''" 
 
 Alligator lucius. 
 (From above.) 
 
 R, radius ; U, ulna ; r, 
 
LIMBS 
 
 131 
 
 aerial (Bats) or aquatic (Pinnipedia, Cetacea, Sirenia) mode of life ; 
 or, again, it may give rise to a prehensile organ. In the latter case 
 (Primates) the radius and ulna, instead of being firmly connected 
 together, articulate with one another, the former being capable 
 
 ScK 
 
 FIG. 111. SKELETON OF THE LIMBS AND TAIL OF A CARINATE BIRD. (The 
 skeleton of the body is indicated by dotted lines. ) 
 
 Sch, scapula ; 7?, coracoid ; St, sternum, with its keel (Or) ; OA, humerus ; Rd 
 ulna ; 77, radius ; HW, carpus ; MH, carpometacarpus ; F, digits ; OS, 
 temur ; T t tibiotarsus ; Fi, fibula ; MF, tarsometatarsus ; 2 1 , Z, digits ; 
 Py, pygostyle. 
 
 of rotation round the latter : thus the rnanus can be brought into 
 a position of pronation or of supination. 
 
 The tibia is the most important bone of the shank, and the 
 fibula often becomes fused with it to a greater or less extent ; the 
 ulna also may unite with the radius. Except in the Cetacea, 
 
 K 2 
 
132 
 
 COMPARATIVE ANATOMY 
 
 Sirenia, Cheiroptera, and certain Marsupialia, a sesamoid bone is 
 developed in the distal tendons of the great extensor muscles of 
 the shank, and is known as the knee-cap or patella. This is 
 already present in certain Lizards and in Birds. 
 
 The carpus and tarsus most nearly correspond with those of 
 Urodeles and Chelonians, and, as in them, certain of the elements 
 
 FIG. 112. A. FORE- ARM AND MANUS or EMBRYO PENGUIN (Eudyptes chry so- 
 come}. (Fourteenth day of incubation.) (After Th. Studer.) (SB is a sesa- 
 moid developed in the tendon of the triceps in this Bird.) B. SHANK AND 
 FOOT OF EMBRYO PENGUIN. (At the same stage.) 
 
 may become fused together. Thus the intermedium and tibiale 
 as a rule unite to form an astragalus, while the fourth and fifth 
 carpals become fused to form the so-called unciform lone, and the 
 corresponding tarsals give rise to the cuboid. A centrale, varying 
 much in form and size, is usually present at an early stage in all 
 five-fingered Mammals, but as a rule it becomes fused later with 
 one, or with two, of the neighbouring carpals generally the 
 
LIMBS 
 
 133 
 
 radiale (e.g., the Gorilla, the Chimpanzee, and Man, though 
 it may persist in the human subject throughout life or may 
 fuse with carpale 2 or 3). In the tarsus the centrale (navicular) 
 remains distinct, and usually lies on the inner border of the foot. 
 
 So much difference of opinion exists with regard to the homologies of the 
 bones of the carpus and tarsus in Mammals, that it is not possible at present 
 to give a satisfactory account of them, or of the additional elements which are 
 often present in the embryo and disappear during development. Thus the 
 pisiform may be a true sesamoid, or may represent an additional uliiar ray, 
 and the calcaneum may or may not be the complete serial homologue of the 
 pisiform. Elements occur occasionally in the carpus and tarsus which are 
 supposed to represent additional radial and tibial rays respectively the 
 so-called prepollex and prehallux (Fig. 113). 
 
 There are typically five complete digits on each foot, but this 
 number may be reduced to four, three, or even one (Figs. 114 and 
 
 // It 
 
 FIG. 113. A, CARPUS, AND B, SKELETON OF THE FOOT OF MAN. (The rudiments 
 of the so-called prepollex and prehallux (-ftf) are represented diagram- 
 mat ically. 
 
 V, ulna ; R, radius ; r, radiale ; i, intermedium ; u, ulnare ; P, pisiform ; ce, 
 centrale, fused with the radiale ; ce' 2 , second centrale, forming the head 
 of tarsale 3 ; 1-5, the carpalia and tarsalia, 4 and 5 being united to form 
 the unciform and cuboid respectively ; Cu /-///, the first to third tarsalia ; 
 c, centrale tarsi (navicular) ; it, intermedio-tibiale = astragalus (As) ; 
 f + p, calcaneum ( = fibulare and pisiform tarsi ?) ; /- F, the metacarpals 
 and metatarsals. 
 
 115), the disappearance taking place in the following order 
 1, 5, 2, 4 : thus in the horse the third is the only complete digit 
 remaining (Fig. 115). The number of phalanges is similar in both 
 hand and foot : in the first digit there are only two, while in the 
 others there are three. An exception to this rule is seen in 
 Cetacea, in which the phalanges are numerous, as in Ichthyosaurus 
 and Plesiosaurus amongst Reptiles. 
 
 It is interesting to trace the reduction which has taken place in the feet 
 of Ungulates in the course of time. Fig. 115 represents successive stages in the 
 
134 
 
 COMPARATIVE ANATOMY 
 
 phylogenetic development of the fore-foot of the Horse, showing how it 
 has been gradually derived from a tetra- or peiitadactyle form; and it has 
 
 recently been ascertained that all 
 
 A 
 
 $ 
 
 a 
 
 these stages are passed through in. the 
 course of ontogeny. In this case the 
 
 I \ I II i third digit becomes greatly enlarged 
 
 |"TjQf jL-lk /. relatively (perissodactyle form), and 
 
 ~ eventually is the only one remaining, 
 while in cloven-footed Ungulates the 
 third and fourth digits are both 
 functional and equally strongly de- 
 veloped (artiodactyle form) and may 
 be united together to form a ' ' can- 
 non-bone," the others becoming 
 gradually reduced. A similar re- 
 duction takes place in the hind- 
 foot, and is here as a rule more 
 rapid. 
 
 Ungulates diverged into Artio- 
 dactyles and Perissodactyles as far 
 back as the Eocene period, but a 
 large series of Tertiary forms shows 
 that they must all have been derived 
 from a common pentadactyle ances- 
 tral form. 
 
 Some of the many other adaptive 
 modifications of the limbs in Mam- 
 mals must also be briefly mentioned. 
 In Bats, the phalanges are greatly 
 elongated to support the wing- 
 membrane ; the hallux as well as 
 the pollex may be opposable amongst 
 the Primates ; the fore-limbs are 
 modified for digging in certain 
 
 FoRE-LiMBOF^4, PIG; B, HYOMOS- Mammals (e.g. Mole); and in the 
 onus ; C, TRAGULUS ; D, ROEBUCK ; Cetacea (see p. 133) and Sireiiia the 
 E, SHEEP ; F, CAMEL. (From Bell, digits are not free, and serve as 
 
 supports for the fin-like paddles. 
 Nails are present on the digits of 
 
 Sireiiia, but have disappeared in the Cetacea, though they can still be 
 recognised in the embryo of toothed Whales. Hind-limbs are absent 
 
 yiYui n 
 
 /- 
 
 * 1 / 1 
 
 j ./ \ 
 
 / i \ 
 
 ,''( 
 
 t 9 
 
 8 l ^ 
 
 3 vj >, 
 
 * X J i4 
 
 ^ y 4 
 
 # 
 
 / 
 
 F 
 
 FIG. 
 
 114. SKELETON 
 
 OF THE LEFT 
 
 FIG. 115. FORE-FOOT OF ANCESTRAL FORMS OF THE HORSE. 1. OROHIPPUS 
 (Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene). 
 4. PROTOHIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene). 
 6. EQUUS. 
 
 in the two last mentioned Orders, but indications of them can be seen even 
 externally in very young embryos of the Porpoise, and rudiments of the thigh 
 and even shank bones occur in the adult in certain Whales (comp. p. 121). 
 
C. MUSCULAR SYSTEM. 
 
 THE muscles, commonly spoken of as flesh, may be divided into 
 two groups A according to the histological character of their 
 elements, which consist of cells elongated to form contractile 
 fibres : namely, into those with smooth and those with transversely- 
 striated fibres. The former are phylogenetically the older, and are 
 to be looked upon as the precursors of the latter. The action of 
 both in causing movements is dependent on the nervous system. 
 
 The smooth or involuntary muscle-fibres preponderate in the 
 vascular system, viscera, and dermis, and are not under the' control 
 of the will ; almost all the striated or voluntary muscles occur in 
 the body-walls and organs of locomotion, and are under the 
 control of the will. 1 The following general statements refer 
 exclusively to the latter kind of muscles, which may, according 
 to their mode of development, be arranged in the following 
 groups : 
 
 (a. Muscles of the trunk, including the 
 coracohyoid (sterno-hyoid) of 
 Fishes and its representatives in 
 
 I. Parietal muscles, de- 
 rived from the meso-. 
 blastic somites. 
 
 higher Vertebrates : these repre- 
 sent the oldest and most primitive 
 part of the muscular system. 
 
 b. Muscles of the diaphragm. 
 
 c. Muscles of the extremities. 
 
 d. Eye-muscles. 
 
 II. Visceral muscles, de-f Cranial muscles, with the exception of 
 rived from the lateral I those included under a and d 
 plates of the mesoblast. [ above. 
 
 In its simplest form, an origin, a belly, and an insertion, may be 
 distinguished in each muscle. The muscles of the trunk are as a 
 
 1 Exceptions are seen in the muscles of the heart, and of the alimentary 
 canal in the Tench. More or less of the anterior and posterior parts of the 
 digestive canal may contain striated fibres in other animals. 
 
136 COMPARATIVE ANATOMY 
 
 rule flat, while those of the extremities have usually an elongated, 
 cylindrical, or prismatic form. In some cases, however, they 
 assume the most various shapes : for instance, there may be more 
 than one origin (bicipital, tricipital, or quadricipital forms), the 
 belly may be double (biventral or digastric form), or the muscle 
 may be saw-shaped, or have its fibres arranged in a single or 
 double series like a feather. 
 
 All the muscles are surrounded by fibrous sheaths, or fascice, 
 by means of which they are more or less firmly connected with 
 one another and with the integument and skeleton. Wherever a 
 marked friction occurs, ossifications (sesamoids) may become de- 
 veloped in the course of a muscle or tendon. 
 
 The differentiation of independent muscles may take place (1) by the 
 separation of the originally single muscle into proximal and distal parts by 
 the formation of an intermediate tendon ; (2) by the splitting of a muscular 
 mass into layers ; (3) by a longitudinal splitting ; or (4) by a fusion of distinct 
 muscles. A muscle may undergo very considerable modification both in form 
 and position by a change of origin and insertion ; and when the action of a 
 muscle becomes unnecessary, it either disappears partly or entirely, or what 
 remains of it contributes to the strengthening of a neighbouring muscle. 
 
 The following important factors must be taken into consider- 
 ation in connection with the muscular system : (1) the homologies 
 of the parts of the skeleton; (2) the relative positions of the 
 neighbouring soft parts; and (3) the nerve- supply. 
 
 Most of the muscles bear a close relation to the skeleton from 
 which they take their origin and into which they are inserted. 
 The integumentary musculature, on the other hand, lies en- 
 tirely in the subcutaneous connective 1 issue, but in Mammals its 
 origin can be traced to the deeper, skeletal muscles : this is 
 most plainly seen in Monotremes. Only slightly developed in the 
 Anamnia, it becomes of great importance in Reptiles and Birds on 
 account of its relations to the scutes, scales, and feathers. It is 
 most highly developed amongst Mammals, where it may extend 
 over the back, head, neck, and flanks as the panniculus carnosus 
 (Echidna, Dasypus', Pinnipedia, Erinaceus, &c.). In Man, only a 
 rudiment of this muscle is found in the shape of the platysma 
 myoides, which extends over the neck and part of the breast and 
 face. 
 
 The action of the integumentary muscles is very different in 
 different Vertebrates. It may (1) serve to roll the body up into a 
 ball (e.g., Hedgehog, Armadillo) ; (2) be connected with a tail 
 adapted for swimming (e.g., Ornithorhynchus) ; (3) serve to erect 
 the integumentary spines (e.g., Echidna) ; or (4) cause local move- 
 ments ("twitching") of the skin (many Mammals). 
 
 The facial muscles, though present in rudiment in the Anamnia, 
 form a marked feature for the first time in Mammals, arising mainly 
 in connection with the platysma myoides, and gradually extending 
 
MUSCULAR SYSTEM 137 
 
 over the face so as to become grouped around the eyes, nose, 
 mouth, and ears. They are supplied by the facial nerve, and attain 
 their greatest development in the Primates, in which certain 
 other facial muscles are derived from the deeper-lying sphincter 
 colli. 
 
 Parietal Muscles. 
 
 A. Muscles of the Trunk. 
 
 In Amphioxus (Fig. 219) the body muscles are made up of a 
 series (60 or more) of lateral muscular segments or myotomes separated 
 by > shaped connective-tissue septa or myocommata, between 
 which the fibres run longitudinally. The myotomes have an 
 alternating arrangement on the two sides. On the ventral region 
 of the anterior two-thirds of the body there is a thin transverse 
 sheet of fibres. 
 
 In Fishes and Dipnoans the myotomes and myocommata are 
 arranged in pairs and consist, on either side of the body, of two 
 portions, a dorsal and a ventral, separated from one another by a 
 connective-tissue septum extending from the axial skeleton to the 
 integument (comp. Fig. 116). 1 The myotomes meet together in 
 the mid-dorsal and mid- ventral lines. 
 
 This primitive metameric arrangement of the lateral muscles of 
 the trunk forms a characteristic feature in Vertebrates, and stands 
 in close relation with the segmentation of the axial skeleton and 
 spinal nerves, the number of vertebrae and pairs of nerves corre- 
 sponding primitively to that of the myotomes. 
 
 The lateral muscles largely retain their primitive relations in 
 Fishes and Dipnoans, but on the ventral side of the trunk, 
 where they enclose the body-cavity (comp. Amphioxus), certain 
 differentiations occur which indicate the formation of the recti and 
 obliqui abdominis of higher types. The dorsal portions of these 
 parietal muscles, as well as the ventral portions in the caudal 
 region, retain the more primitive relations. 
 
 Amphibia. In Urodeles (Figs. 116 and 117) primary and 
 secondary ventral trunk-muscles can be distinguished, and both of 
 these groups, like the dorsal muscles, are segmented. The former 
 group consists of internal and external obliqui and recti. The 
 secondary muscles arise by delamination from the primary, and 
 give rise to a superficial external oblique, a superficial rectus, a 
 transversalis, and a subvertebralis. These, however, only attain 
 importance in caducibranchiate forms, in which they become 
 marked during metamorphosis, and the primary musculature then 
 
 1 This septum is not present in Myxinoids, and is absent in Petromyzon and 
 Lepidosteus posteriorly to the gills. 
 
138 
 
 COMPARATIVE ANATOMY 
 
 undergoes more or less reduction. Thus various conditions of the 
 ventral musculature are found amongst Urodeles. 
 
 In the Anura, on the other hand, both primary and secondary 
 muscles present a marked uniformity and relative simplicity ; in 
 the adult they give rise to a segmented rectus, an obliquus 
 externus, and a transversal is, as well as to a cutaneus abdominis 
 derived from the external oblique. No trace of an internal oblique 
 can be seen in the adult. 
 
 Reptiles. In Reptiles, the lateral muscles of the trunk attain 
 a much higher grade of development. This is to be accounted 
 
 FIG. 116. THE MUSCULATURE OF Siredon pisciformis. (From the side.) 
 
 LI, lateral line ; D, dorsal, and V, ventral portion of caudal muscles ; EM, dorsal 
 portion of lateral muscles of the trunk ; O, 0, outer layer of the external 
 oblique muscle, arising from the lateral line, and extending to the fascia, F ; 
 at * a piece of this layer is removed, exposing the inner layer of the muscle 
 (Ob] ; at Re the oblique fibres of the latter pass into longitudinal fibres, 
 indicating the beginning of the differentiation of a rectus abdominis ; at Re 1 
 the rectus-system is seen passing to the visceral skeleton ; Me, fibrous parti- 
 tions between the myotomes of the dorsal portion of the lateral muscles ; T, 
 temporal ; Ma, masseter ; Dg, digastric ; Mh l , mylohyoid (posterior portion) ; 
 Ce, external ceratohyoid muscle ; Lv, levator arcuum branchialium ; ftt , 
 levator branchiarum ; Cph, cervical origin of the constrictor of the pharynx ; 
 Th, thymus ; Lt, latissimus clorsi ; Ds, dorsalis scapulse ; Cu, cucullaris ; 
 SS, suprascapula ; Ph, procoraco-humeralis. 
 
 for by the more perfect condition of the skeleton, more especially of 
 the ribs and pectoral arch. The ribs and intercostal muscles now 
 play an important part in respiration, and changes, necessitated by 
 the more important development of the lungs, are thus brought 
 about. 
 
 The distinction between thoracic and abdominal regions becomes 
 gradually more plainly marked, and distinct external and internal 
 intercostal muscles are now differentiated. In the lumbar region 
 the ribs become gradually withdrawn from the muscles lying 
 
MUSCULAR SYSTEM 
 
 139 
 
 between them; the]muscles thus lose their, intercostal character, 
 and form connected sheets, extending between the last pair of ribs 
 
 FIG. 117. THE MUSCULATURE OF Siredon pisciformis. (Ventral view.) 
 
 0, outer layer of the external oblique, passing into the fascia, which is shown 
 cut through at F ; Oh, inner layer of the same muscle ; He, rectus abdominis, 
 passing into the visceral musculature (sternohyoid) at Re 1 , and into the pector- 
 alis major at P ; Mh, Mh l , anterior and posterior portions of the mylohyoid, 
 which is cut through in the middle line, and removed on the left side, so as 
 to show the proper visceral musculature ; Ce, Ci, Ci 1 , external and internal 
 ceratohyoid : the former is inserted on to the hyoid (Hy) ; Add, adductor 
 arcuum branchialium ; C, constrictor arcuum branchialium ; Cph, portion of 
 the constrictor of the pharynx, arising from the posterior branchial arch ; 
 Dp, depressores branchiarum ; Gh, genio-hyoid ; Ph, procoraco-humeralis ; 
 Spc, supracoracoideus ; Cbb, coraco-branchialis brevis ; Clo, cloaca ; La, 
 linea alba. 
 
 and the pelvic arch (e.g., the quadratus lumlorum, which lies close 
 against the vertebral column). 
 
HO COMPARATIVE ANATOMY 
 
 The rectus abdominis, which is always well developed, but does 
 not extend anteriorly to the sternum, becomes divided into three 
 portions, a ventral, an internal, and a lateral. 
 
 While no important differentiation is noticeable in the dorsal por- 
 tion of the lateral body-muscles in Urodeles, a marked subdivision 
 of these muscles is seen in Reptiles. In them may be distinguished 
 a longissimus, an ilecostalis, inter spinales, semispinales, multifidi, 
 splenii, and levatores costarum, together with the scaleni, certain 
 of which belong to the last-mentioned group, and others to the 
 intercostal muscles. 
 
 The muscles of the main part of the tail retain primitive rela- 
 tions similar to those seen in Fishes : at the root of the tail and in 
 the cloacal region, however, new muscles become differentiated. 
 
 Birds. In Birds the primitive character of the trunk-muscles 
 has disappeared far more than in Reptiles. This is mainly to be 
 accounted for by the excessive development of the muscles 
 of the anterior extremity the pectoralis major more particu- 
 larly, and the corresponding backward extension of the breast- 
 bone. 
 
 External and internal oblique muscles are present, but only 
 slightly developed : this is more particularly true of the internal, 
 which appears to be undergoing degeneration. No trace of a 
 transversalis can be distinguished ; but, on the other hand, a paired, 
 unsegmented rectus is present. 
 
 External and internal intercostals are well developed, and a 
 triangularis sterni appears for the first time on the inner surface 
 of the sternal ends of the ribs. 
 
 The dorsal portion of the trunk musculature is only slightly 
 developed in the region of the trunk, though very strongly marked 
 in the neck. 
 
 All these modifications in Birds seem to be accounted for by 
 the specialisation of the mechanisms for flight and respiration, to 
 assist which the greatest possible number of muscles are brought 
 into play and thereby influence the whole organism : an essential 
 difference is thus brought about between Birds and Reptiles. 
 
 Mammals. Three lateral abdominal muscles are always 
 present in Mammals, an external and internal oblique and a trans- 
 versalis. In many cases, more particularly in Tupaia and in Lemurs, 
 the external oblique possesses tedinous intersections, thus indicat- 
 ing its primitive segmental character; but in general all these 
 muscles consist of broad uniform sheets. Towards the middle line 
 they pass into strong aponeuroses, which ensheath the rectus 
 abdominis. The latter consists of a single band on each side and 
 possesses a varying number of myocommata ; it is no longer con- 
 nected with the axial muscles of the neck belonging to the same 
 system (sternohyoid, sternothyroid, &c.) as is the case in Urodeles, 
 
MUSCULAR SYSTEM 141 
 
 for the sternum is always interposed between them, as it is in 
 the Sauropsida. 
 
 In Monotremes and Marsupials, a strong pyramidali* muscle 
 lies on the ventral side of the rectus abdominis. It arises from 
 the inner border of the "marsupial bones" (epipubes, p. 121) and 
 may extend forwards as far as the sternum. In the higher 
 Mammals, where the epipubes are absent, the pyramidal is usually 
 becomes greatly reduced or entirely lost. Traces of it are, however, 
 commonly to be met with even in the Primates, and always arise 
 from the anterior border of the pubis, right and left of the middle 
 line. 
 
 The external and internal oblique muscles are represented in 
 the thoracic region in Mammals, as in the Sauropsida, in the form 
 of external and internal intercostals. 
 
 What has been said above as to the differentiation of the dorsal 
 portion of the trunk-muscles in Reptiles applies also essentially 
 to Mammals. 
 
 The greater number of the muscles in connection with the 
 external genital organs become differentiated from the primitive 
 sphincter cloacce : the origin of the others is not known. 
 
 B. Muscles of the Diaphragm. 
 
 A complete diaphragm dividing the coelome into thoracic and 
 abdominal cavities occurs only in the Mammalia. It is dome- 
 shaped and muscular, its muscles arising from the vertebral column, 
 ribs, and sternum. The diaphragm is of great importance in 
 respiration, as it allows of a lengthening of the thoracic- cavity in 
 a longitudinal direction. It is supplied by a phrenic nerve, arising 
 from one or more (3rd to 6th) of the cervical nerves ; and usually 
 consists of a central tendon, perforated by the cesophagus*and post- 
 caval vein, and of muscular fibres radiating from this to the 
 periphery and forming dorsally two strong " pillars of the dia- 
 phragm." In some cases (e.g., Echidna, Phoca3na) the diaphragm 
 is entirely muscular. 
 
 Amongst the Sauropsida, a partition is present between the 
 pleural and peritoneal cavities in Chelonians, and is still more 
 marked in Crocodiles and Birds x : this is connected with the 
 ribs by muscular fibres. It, however, does not enclose the peri- 
 cardium, which, as in the Anamnia, lies in the general peritoneal 
 cavity. 
 
 The evolution of the mammalian diaphragm is not yet tho- 
 roughly understood. 
 
 1 In Birds, two entirely different structures have been described as a 
 diaphragm. (See under Air-sacs.) 
 
142 COMPARATIVE ANATOMY 
 
 c. Muscles of the Appendages. 
 
 The most primitive condition of the muscles of the extremities 
 is met with in Fishes and Dipnoans, in which the musculature of 
 each surface of the fin forms a more or less uniform mass which 
 may become differentiated into layers. Everything goes to prove 
 that all the muscles of the appendages are to be looked upon 
 primarily as derivatives of the lateral muscles of the trunk, i.e., 
 of the myotomes; and although in the Amniota they have 
 apparently an independent origin, this is probably only due to an 
 abbreviation of development. 
 
 Two principal groups of appendicular muscles may always be 
 distinguished : one lying in the region of the pectoral and pelvic 
 arches, dorsally and ventrally, the other in the free extremity. In 
 Fishes and Dipnoans the latter consist essentially of elevators, 
 adductors, and depressors of the fins ; while from the Amphibia 
 onwards, in correspondence with the more highly-differentiated 
 organs of locomotion, considerable complication is seen, and 
 there is a much more marked separation into individual muscles 
 corresponding with the different sections of the extremity. Thus 
 elevators, depressors, rotators, flexors, extensors, and adductors are 
 present in connection with the upper arm and thigh, fore -arm and 
 shank, and hand and foot, and the digits are also moved by a 
 highly-differentiated musculature. The number of muscles gradu- 
 ally increases in passing from the Urodela through the Sauropsida 
 to the Mammalia. 
 
 When, as in the Primates, the anterior extremity is con- 
 verted into a prehensile organ, new groups of muscles appeal- 
 known as pronators and supinators. The former are derived from 
 flexors, the latter from extensors. 
 
 D. The Eye -Muscles. 
 
 (These will be treated of in connection with the organ of 
 vision.) 
 
 Visceral Muscles. 
 
 Fishes. Considerable differences exist in the visceral mus- 
 culature of Fishes. 1 In Elasmobranchs, Furbringer classifies these 
 muscles as follows : 
 
 A. Cranial muscles (consisting originally of transverse or 
 circular fibres) supplied by the V th , VII th , IX th , and X th 
 cerebral nerves. 
 
 1 In Cydostomes there is a remarkable transformation of the cranio-visceral 
 musculature in correspondence with their peculiar cranial skeleton (suctorial 
 apparatus) and branchial basket. 
 
MUSCULAR SYSTEM 143 
 
 1. Constrictor arcuum visceralium, incl. constrictor superficialis 
 
 dorsalis and ventralis. 
 
 Innervation, 
 
 Levator labii superioris ^ 
 
 , , maxillae , , 
 ,, palpebrse nictitantis l ) 
 
 rostri 
 
 ,, hyomandibularis y VII. 
 
 Depressor rostri 
 
 ,, mandibularis and hyomandibularis, 
 
 Interbranchiales IJt 
 
 Trapezius 
 
 risj 
 
 2. Arcuales dorsales IX, X. 
 
 3. Adduetores, incl. adductor mandibulaj .... V. 
 
 and adductores arcuum branchialiuni. . . IX, X. 
 
 B. Spinal muscles (originally longitudinal), divided, like the 
 trunk-muscles, into myotomes. Supplied by the spino- 
 occipital ( = the " ventral roots" of X) and spinal 
 nerves. 
 
 (a) Epibranchial spinal muscles, dorsal to visceral skeleton. 
 
 Innervation. 
 
 4. Subspinalis . ... Spino-occipital nerves. 
 
 ( Spino-occipital nerves, 
 
 5. Interbasales. . . < as well as the first 
 
 ( spinal nerve. 
 
 (6) Hypobranchial spinal muscles, ventral to visceral skeleton. 
 
 r Spinal nerves, and 
 
 6. Coraco-arcuales, iucl. coraco-bran- I part ly the last one or 
 
 chiales, coraco-hyoideus, and < more of the spino . 
 coraco-mandibularis . . { occipital nerves. 
 
 The structure of the cranio-visceral musculature of Ganoids and Teleosts 
 differs considerably from that roughly sketched out above, so that the 
 different groups of muscles must be arranged in an entirely different manner. 
 Thus in Teleostei the following divisions may be distinguished : (1) Muscles 
 of the jaw, (2) muscles of the dorsal, and (3) muscles of the ventral ends of 
 the visceral arches. Each of these groups may again be sub-divided, but 
 further details about their arrangement, which is often very complicated, 
 cannot be given here. The visceral muscles of Polypterus are of especial 
 interest, as they show an intermediate condition between those of Elasmo- 
 branchs and Urodeles. 
 
 Amphibia. It is to be expected, a priori, that the muscula- 
 ture of the visceral skeleton should be more highly developed in 
 branchiate than in air-breathing Amphibians ; we thus find 
 that in the former more primitive relations are met with, connect- 
 
 1 This muscle has therefore nothing to do with the ether eye-muscles. 
 
144 COMPARATIVE ANATOMY 
 
 ing them with lower forms, while in the latter a greater modification, 
 or rather reduction, of these muscles takes place. 
 
 Between the two rami of the lower jaw is situated a muscle with 
 transverse fibres (the mylohyoid), supplied by the third division of 
 the trigeminal and the facial nerve ; this represents the last rem- 
 nants of the constrictor muscle of Fishes. As the elevator of the 
 floor of the mouth, it stands in important relation to respiration and 
 deglutition, and is retained throughout the rest of the Vertebrata 
 up to Man (Figs. 116, 117). 
 
 A continuation of the trunk-musculature (the omo-, sterno-, and 
 genio-hyoid) provided with tendinous intersections, lies above the 
 mylohyoid (Fig. 117). These muscles, which serve to pull the 
 visceral skeleton forwards and backwards, are supplied by the first 
 and second spinal nerves. 
 
 In contrast to Fishes, there is in Amphibians a definite differen- 
 tiation into muscles of the tongue, that is, into a liyogiossus and a 
 genioglossm ; but these also must be considered as having been 
 derived from the anterior end of the ventral muscles of the trunk ; 
 they are present in all Vertebrates, from the Amphibia onwards, 
 and are supplied by the hypoglossal ( = the first spinal nerve of 
 Amphibians) . 
 
 In the Perennibranchiata and in Salamander larvae the muscles 
 of the hyoid and of the visceral arches may, as in Fishes, be 
 divided into a ventral and a dorsal group ; the latter disappears in 
 adult Salamanders and Anurans, only the ventral persisting. Their 
 function is to raise and depress the branchial arches, as well as to 
 draw them forwards and backwards. To these may be added 
 constrictors of the pharynx, as well as (in branchiate forms) 
 levators, depressors, and adductors of the external gill filaments 
 (Figs. 116 and 117). They are innervated by the vagus and 
 glossopharyngeal. 
 
 The jaw-muscles include a depressor (digastric, or biventer 
 mandibulce, Fig. 116), supplied by the facial nerve, and 
 elevators of the lower jaw (masseter, temporal, and pterygoid 
 muscles), supplied by the third division of the trigeminal. All 
 these muscles, which may be derived from the adductor of the. 
 mandible of Elasmobranchs and Ganoids, arise from the auditory 
 region of the skull. 
 
 Amniota. With the simplification of the visceral skeleton in 
 Amniota there is a considerable reduction of the musculature 
 belonging to it. All muscles connected with branchial respiration 
 are ofcourse wanting, and the ventral trunk-muscles, as mentioned 
 above, are always interrupted in their forward extension by the 
 sternum and pectoral arch. At the same time, the muscles along 
 the neck and on the floor of the mouth met with in Amphibia are 
 present here also ; they are, a mylo-, sterno-, omo-, and genio- 
 hyoid, as well as a hyoglossus and genioglossus. To these may 
 
MUSCULAR SYSTEM 145 
 
 l>e also added a sterno-thyroid, from which a thyro-hyoid is con- 
 tinued forwards. 
 
 The stylo-hyoid, stylo-glossus, and stylo-pharyngeus of Mam- 
 mals, arising from the styloid process and stylo-hyoid ligament and 
 undergoing numerous variations, are peculiar to Mammals. They 
 are supplied partly by the facial nerve, partly by the glossopharyn- 
 geal, and act as retractors of the tongue and levators of the pharynx 
 and hyoid. 
 
 The muscles of the jaws resemble those of Amphibia, although, 
 especially in the case, of the pterygoids, 1 they are much more 
 sharply differentiated, and are throughout more strongly developed. 
 
 1 For the tensor tympani and stapedius muscules, see under Auditory Organ. 
 
D. ELECTRIC ORGANS. 
 
 ELECTRIC organs are present in certain Fishes, being most 
 strongly developed in certain Rays (Torpedinida3, e.g., Torpedo,. 
 Eypnos) found in the Atlantic Ocean and various southern seas, 
 in- a South American Eel (Gymnotus clectricus) and in an African 
 Siluroid (Malopterurus electricus). Gymnotus possesses by far 
 the strongest electric power, next to it comes Malopterurus, 
 and then Torpedo. The electric batteries of these three Fishes 
 
 are situated in different parts of 
 the body : in the Torpedinidse they 
 have the form of a broad mass, 
 extending throughout the substance 
 of the part of the body lying be- 
 tween the gill-sacs and the pro- 
 pterygium on either side of the 
 head (Fig. 118); in Gymnotus they 
 He in the ventral region of the 
 enormously long tail (Fig. 119), 
 that is, in the position usually 
 occupied by the ventral portions of 
 the great lateral muscles ; and 
 finally, in Malopterurus, the electric 
 organ extends between the skin and 
 muscles round almost the entire 
 circumference of the body, thus 
 enclosing the Fish like a mantle : 
 it is especially strongly developed 
 along the sides. 
 
 The electric power of those 
 Fishes which were formerly known 
 as " pseudo-electric " has now been 
 fully demonstrated, though it is 
 much feebler than in the forms 
 described above. To this category 
 belong all the Rays, excluding the 
 
 Torpedinidse, the various species of Mormyrus, and Gymnarchus 
 (both the latter genera belonging to the Teleostei). In all these, 
 the electric organs lie on either side of the end of the tail and have 
 a metameric arrangement like that of the caudal muscles ; in the 
 
 FIG. 118. Torpedo marmorata, 
 WITH THE ELECTRIC ORGANS (E) 
 
 EXPOSED. 
 
 8, skull ; Sp, spiracle ; KK, gills ; 
 Au, eye. 
 
ELECTRIC ORGANS 
 
 147 
 
 Mormyridse, for example, there is on each side an upper and lower 
 row of electric organs. 
 
 The electric apparatus in all the above-named Fishes is to be 
 regarded from the same point of view both as concerns its mode 
 of development and its 
 anatomical relations : 
 all electric organs are 
 to be looked upon as 
 consisting of metamor- 
 phosed muscular fibres 
 and the nerve-endings 
 belonging to them as 
 homologues ofthemotor 
 end-plates which are 
 ordinarily found on 
 muscles. 
 
 As regards the 
 structure of the elec- 
 tric organs, the same 
 essential arrange- 
 ments are met with 
 in all : the details of 
 their histology ami 
 physiology cannot be 
 entered into here. The 
 framework is formed 
 of fibrous tissue en- 
 closing numerous eel Is, 
 which, running partly 
 longitudinally, partly 
 transversely through 
 the organ, gives rise 
 to numerous poly- 
 gonal or more or less 
 rounded chambers or 
 compartments. These 
 latter are arranged in 
 rows, either along the 
 longitudinal axis of 
 the body (Gymnotus, 
 Malopterurus) or in a 
 dorso-ventral direction 
 (Torpedo), forming definite prismatic columns (Fig. 120). 
 
 Numerous vessels and nerves ramify in the connective-tissue 
 lying between these compartments, the nerves being enclosed in 
 thick sheaths, and having a different origin in the different 
 forms. In Torpedo, in which the electric organs probably arise 
 in connection with the great adductor muscle of the mandible 
 
 L 2 
 
 FIG. 119, A and B. THE ELECTRIC ORGAN OF 
 Gymnotus electricus. (B, from a preparation by 
 A. Ecker.) 
 
 H, skin ; Fl, fin ; DM, DM 1 , dorsal portions of the 
 great lateral muscles, seen partly in transverse, 
 partly in longitudinal, section ; VM, VM 1 , 
 ventral portions of ditto ; E, the electric organ, 
 seen in transverse section at E (B), and from 
 the side at E 1 ; WS, vertebral column from 
 the side, and the spinal nerves, and WS 1 , 
 in transverse section ; LH, posterior end of 
 body cavity ; Sep, median longitudinal fibrous 
 septum between the left and right electric 
 organ and lateral trunk-muscles ; A, anus. 
 
148 
 
 COMPARATIVE ANATOMY 
 
 hundred nerves 
 
 and the constrictor of the gill-arches, the nerves arise from 
 the " electric lobe " of the medulla oblongata, a single branch 
 coming also from the trigeminal nerve ; in all pseudo- 
 electric Fishes, as well as in Gyrnnotus, in which over two 
 pass to the electric organ, they arise from the 
 spinal cord, and are probably in close relation 
 with the ventral cornua of the latter, which 
 are particularly well developed in the last-named 
 Fish. It is remarkable that the electric nerves 
 of Malopterurus arise on each side from a single 
 enormous lens-shaped nerve-cell, which, lying in 
 the neighbourhood of the second spinal nerve, is 
 continued into a very large primitive-fibre which 
 passes towards the end of the tail, dividing as 
 it goes. This fibre is invested by a thick sheath. 
 
 PIG. 120. ELEC- 
 TRIC PRISMS OF 
 Torpedo mar- 
 morata. (Semi- 
 diagrammatic. ) 
 
 Experiments have shown that all Electric Fishes are 
 proof against the electric current, with the limitation that 
 muscles and nerves even the electric nerves themselves 
 separated out from the body are capable of being excited 
 by the current. ' ' The last and most important question with regard to the 
 Electric Fishes is naturally concerning the mechanism whereby the electric 
 plates become temporarily charged with electricity. The reply to this ques- 
 tion, although probably not so difficult a one as that relating to the mechanism 
 of muscular contraction, is still far from being answered " (Du Bois-Reymond). 
 The only thing that can be stated with certainty is, that the electromotive 
 force is under the influence of the will. 
 
E. NERVOUS SYSTEM. 
 
 THE nervous system, as already mentioned in the Introduction 
 (p. 5), arises from the epiblast, and the first parts to become 
 differentiated histologically are the nerve-cells (ganglion-cells), 
 from which nerve-fibres arise later and serve as conductors of 
 nervous impulses. The most important constituent of the nerve- 
 fibre is a central axis-cylinder or axis-fibre, and in those nerve-fibres 
 which are spoken of as medullated this is surrounded by a highly 
 refractile, fat-like substance (myelin), which forms the medullary 
 sheath. In certain (non-medullated) nerve-fibres this sheath is 
 wanting, but the two kinds of fibres are not sharply marked off 
 from one another, either locally or genetically : a fibre may be 
 medullated in one part of its course, and non-medullated in another. 
 Externally each nerve-fibre is enclosed by a delicate sheath, the 
 neurilemma. 
 
 Part of the epiblastic tissue which forms the nervous system of 
 the embryo does not become transformed into nervous tissue, but 
 gives rise to a supporting and connecting framework the neuroglia\ 
 and externally, investing membranes as well as blood and lymph- 
 vessels, are formed from the mesoblast. 
 
 The nervous system consists of central and peripheral portions 
 (Fig. 121). The central part (brain and spinal cord) is the first 
 to arise, and is formed as a direct product of the epiblast ; the 
 peripheral portion (cerebral, spinal, and sympathetic nerves] becomes 
 established later. 
 
 1. THE CENTRAL NERVOUS SYSTEM. 
 
 The first indication of the central nervous system is a longi- 
 tudinal furrow (medullary groove, Fig. 6, A) which appears on the 
 dorsal side of the embryo and gradually becomes converted into 
 a tube by the meeting of its edges ; this tube, consisting ori- 
 ginally of epithelial cells like the epiblast from which it arises, 
 then becomes separated from the epiblast and gives rise to the 
 hollow medullary cord l (Fig. 6, B), in which nerve-cells and fibres 
 soon become differentiated; it comprises a more expanded an- 
 terior and a longer and more slender posterior section. From the 
 former arises the brain, from the latter the spinal cord. 
 
 1 The cord is at first solid in Cyclostomes, Teleosts, and bony Ganoids, 
 cavity being formed secondarily. 
 
150 
 
 COMPARATIVE ANATOMY 
 
 - Wo 
 
 FIG. 121. THE ENTIRE NERVOUS SYSTEM or THE FROG. (After A. Ecker.) 
 From the ventral side. 
 
 He, cerebral hemispheres (prosencephalon) ; Lop, optic lobes (mesencephalon), 
 M, spinal cord ; Ml to M10, spinal nerves, which are connected at SM by 
 branches (rami communicantes) with the ganglia (Si to S10) of the sympathetic 
 (8) -, No, femoral nerve ; Ni, sciatic nerve ; / to X, first to tenth cranial 
 nerves ; G, ganglia of the vagus ; Vg, Gasserian ganglion ; o, eye ; N, nasal 
 sac ; Va to Ve, the different branches of the trigeminal ; F, facial nerve ; 
 Vs, connection of the sympathetic with the Gasserian ganglion ; XI to X4, 
 the different branches of the vagus. Some of the fibres of the sjanpathetic 
 should be shown accompanying the vagus peripherally. 
 
MEMBRANES OF BRAIN AND SPINAL CORD 151 
 
 In an early stage of development the lurnen of the medullary 
 cord is primitively continuous posteriorly with that of the primary 
 intestine (neurenteric canal). This connection, however, soon dis- 
 appears, and the cord then consists of a cylindrical or more or less 
 flattened hollow cord with thick walls, the cavity of which is lined 
 by ciliated epithelium and expands in front to form the ventricles of 
 the brain. This cavity becomes greatly reduced later, and in the 
 spinal cord is spoken of as the central canal. 
 
 Membranes of the Brain and Spinal Cord. 
 
 The enveloping membranes of the brain and spinal cord arise 
 by the differentiation of a connective-tissue layer lying between the 
 central organs of the nervous system and the surrounding skeletal 
 parts. In Fishes, only two membranes are distinguishable : one, 
 
 FIG. 122. BRAIN MEMBRANES OF MAN. (After Schwalbe.) 
 
 DM, dura mater ; SR, sub-dural (arachnoid) space ; A, sub-arachnoid space ; PM, 
 pia mater ; OR, gray cortical substance of the brain. 
 
 the dura mater, lining the inner surface of the cerebro-spinal 
 canal, and the other, or pia mater, investing the brain and spinal 
 cord. The latter represents also the arachnoid of higher Verte- 
 brates, which is not here differentiated as a separate membrane. 
 The dura mater conveys vessels to the walls of the cerebro-spinal 
 canal that is, to the perichondrium or periosteum, while the pia 
 mater, which is much richer in blood-vessels, has to do with the 
 nutrition of the nervous axis. The dura mater consists of two 
 lamellae, which, however, only remain distinct along the whole 
 central nervous system in the lower Vertebrata. In higher Verte- 
 brates, its double nature persists only in the region of the vertebral 
 column, the two layers becoming fused in the cranial portion. As 
 in most Fishes the brain by no means fills the cranial cavity, a large 
 lymph-space lies between the dura and pia mater; this cor- 
 responds to the so-called sub-dural space of terrestrial Vertebrates. 
 
152 
 
 COMPARATIVE ANATOMY 
 
 A differentiation of the primary vascular membrane of the 
 brain and spinal cord into pia mater and arachnoid takes place 
 from the Amphibia onwards, and these two layers become 
 separated in those places where there are deep depressions be- 
 tween the individual parts of the brain ; the deeper of these (pia) 
 adheres closely to the brain, and also penetrates into the ventricles 
 in the form of telce choroidece and plexus choroidei, while the 
 
 superficial one (arachnoid) simply bridges 
 over the depressions (Fig. 122). A lymph - 
 sinus (sub-arachnoid space) is thus de- 
 veloped between the two in the Saurop- 
 sida and Mammalia, but this never reaches 
 such an independent differentiation as 
 does the sub-dural (arachnoid) space. 
 
 1. The Spinal Cord. 
 
 The spinal cord is at first of a uniform 
 diameter throughout, but as a richer 
 nerve-supply becomes needed for the 
 extremities, it exhibits in these regions 
 definite swellings the l)racliial and 
 lumbo-sacral enlargements (Fig. 123). 
 The cord originally extends along the 
 whole length of the neural canal, but 
 its growth is usually less rapid than that 
 of the vertebral axis, so that eventually 
 it is considerably shorter than the latter. 
 In such cases (e.g. Primates, Cheirop- 
 tera, Insectivora, Anura, Figs. 121 and 
 123) it passes at its posterior end 
 into a brush -like mass of lumbo-sacral 
 nerves, the so-called cauda equina, lying 
 within the neural canal. A prolongation 
 of the spinal cord nevertheless extends 
 far back amongst these as a thin thread- 
 like appendage, tlwfilum terminate. 
 
 The bilaterally-symmetrical form of 
 the spinal cord is pronounced by the 
 presence of longitudinal fissures running 
 along it dorsally and ventrally ; 1 and if one imagines the points of 
 exit of the dorsal and ventral nerve-roots to be respectively con- 
 nected together by a longitudinal line, each half of the spinal 
 cord would thus be divided into three columns, a dorsal, lateral, 
 and ventral. 
 
 1 The ventral fissure is not always present, and the so-called dorsal fissure, 
 which is formed by obliteration of the greater part of the primitive central canal, 
 is better described as the dorsal septum. 
 
 FIG. 123. DIAGRAMS OF THE 
 SPINAL CORD AND ITS 
 NERVES. In A the cord 
 passes to the end of the 
 tail, and at B it ends more 
 anteriorly and passes be- 
 hind into a filum termin- 
 ale (F.t). 
 
 M.o, medulla oblongata; PC, 
 cervical nerves ; Pb, bra- 
 chial nerves ; P.lh, thor- 
 acic nerves ; PI, lumbo- 
 sacral nerves ; Ce, cauda 
 equina. 
 
THE BRAIN 153 
 
 As regards its minute structure, two parts can be distinguished 
 in the spinal cord, & white substance, consisting of nerve-fibres only, 
 and a gray substance, composed of nerve-cells as well as fibres. Their 
 relative positions vary in the different animal groups, as well as in 
 the different regions of the cord ; the white substance, however, 
 has typically a more peripheral, the gray a more central position, 
 the latter surrounding the central canal and usually presenting a 
 pair of dorsal and ventral cornua in transverse section. 
 
 2. The Brain. 
 
 Before the medullary groove becomes closed, the anterior ex- 
 panded part of the medullary tube presents three swellings, which 
 are spoken of as the primary fore-, mid-, and hind-brain, or anterior, 
 middle, and posterior cerebral-vesicles (Fig. 124) ; the cavities of 
 the vesicles (ventricles) are in direct connection with the central 
 canal of the spinal cord. Botli the primary fore- brain and hind- 
 brain then become differentiated, each into two parts, and thus five 
 
 FIG. 124. DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS 
 
 SYSTEM. 
 
 G, brain, with its three primary vesicles, 7, //, /// ; R, spinal cord. 
 
 divisions of the brain may be distinguished. Counted from before 
 backwards these are : prosencephalon (secondary fore-brain), thala- 
 mencephalon (primary fore-brain), mesencephalon (mid-brain), meten- 
 cephalon (secondary hind-brain), and myelencephalon (primary 
 hind-brain). The prosencephalon usually gives rise to a pair of 
 lobes, the cerebral hemispheres, and in the mid-brain a pair of 
 optic lobes or corpora bigemina become differentiated dorsally, and 
 two longitudinal bands, the crura cerebri, ventrally. The meten- 
 cephalon is usually spoken of as the cerebellum, and the myelen- 
 cephalon as the medulla oblongata. 
 
 From the base of the prosencephalon or hemispheres paired 
 olfactory lobes (rhinencephala} are given off anteriorly, and the floor 
 or central part of each hemisphere becomes thickened to form a 
 large " basal ganglion," the corpus striatum, while its peripheral 
 part is distinguished as the " mantle" or pallium (Fig. 125). 
 
 The relative development and differentiation of the pallium 
 stands in close relation to the mental development of the animal, and 
 reaches its greatest perfection in Mammals, especially in Man. In 
 certain Fishes the pallium remains partially or entirely non- 
 nervous, retaining its primitive epithelial character, and a layer 
 
154 COMPARATIVE ANATOMY 
 
 of cortical gray matter is only distinctly differentiated from Rep- 
 tiles onwards. No regular series of gradations can, however, be 
 traced in this respect in the various groups. 
 
 Connecting the two lateral halves of the fore-brain are certain 
 transverse bands .of nerve-fibres or commissures. An anterior 
 commissure is present in the posterior region of the secondary fore- 
 brain, a middle in the primary fore-brain, and a posterior in 
 the anterior part of the mid-brain. In addition to these, others 
 may be developed between the hemispheres, but only attain im- 
 portance in Mammals : they are known as the corpus callosum and 
 ihefornix. 
 
 The outer surface of the hemispheres in all Vertebrates below 
 the Mammalia is more or less smooth : in the latter Class, convolu- 
 tions (gyri) separated by fissures (sulti) may be present. The 
 
 FIG. 125. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE SKULL AND 
 BRAIN OF AN (IDEAL) VERTEBRATE EMBRYO. (In part after Huxley.) 
 
 Be, basis cranii ; Ch, notochord ; SD, roof of skull ; NH 1 , nasal cavity ; VH, 
 secondary fore-brain (prosencephalon), showing the corpus striatum (Cs) at the 
 base, and the olfactory lobe (Off) anteriorly ; ZH, thalamencephalon (primary 
 fore-brain), which has given rise dorsally to the pineal body (epiphysis, Z), 
 and ventrally to the infundibulum (/), to which the pituitary body (hypo- 
 physis, H] is attached : anterior to this is seen the optic nerve (Opt], arising 
 from the optic thalamus ( Tho) ; HC, posterior commissure; MH, mid-brain 
 (mesencephalon) ; HH, cerebellum (metencephalon, secondary hind-brain) ; 
 NH, primary hind-brain (myelencephalon) ; Cc, central canal of spinal cord. 
 
 convolutions consist of folds of the gray cortical substance, which 
 cause a greater or less increase of the superficial area. 
 
 From the thalamencephahn, the ventricle of which is walled-in 
 anteriorly by the lamina terminalis, the following structures arise 
 (Fig. 125) : the optic thalami, formed as thickenings of its lateral 
 walls; the primary optic vesicles, arising as paired ventro-lateral 
 outgrowths from \vhich the optic nerves and retina are derived 
 later ; the pineal apparatus, developed as tube-like outgrowths of 
 thereof; and finally, the infundibulum, formed as a funnel-like 
 extension of the floor, together with a part of the pitnit&ry body 
 (hypophysis). The other portion of the pituitary body arises by a 
 gradual pinching off of the epithelium of the primary oral involu- 
 tion (stomodceum, p. 5, and Fig. 126), which gives rise to a 
 gland-like structure, and other parts (saccus vasculosus, &c.) arise 
 in close connection with it. 
 
THE BRAIN 
 
 155 
 
 The pineal apparatus consists of the epiphysis or pineal organ 
 proper, which persists in a more or less rudimentary condition in 
 all Vertebrates, and of a more anterior outgrowth which may be 
 called the parietal organ, arising from the epiphysis or indepen- 
 dently from the roof of the thalamencephalou ; the latter organ 
 becomes atrophied in the majority of Vertebrates. Each of these 
 structures represents a vestigial sensory organ, and in certain cases 
 may retain to a greater or less extent the character of a median eye 
 possibly in some degree comparable to that of Tunicates. 1 
 
 Certain facts seem to indicate that both organs arose primitively in a 
 paired manner. Accessory vesicles occur occasionally in young Slow- 
 worms (Ang-uis), in which as many as two or even three rudimentary vesicles 
 may be present behind the pineal organ. 
 
 FIG. 126. MEDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A NEWLY- 
 HATCHED LARVA OF Petromyzon planeri. (Mainly after Kupfer.) 
 
 J".b, f ore -brain ; m.b, mid-brain ; h.b, hind-brain; ep, epiphysis; hp, hypophysis; 
 st, stomodeeum ; al, endodermic alimentary cavity ; ch, notochord. 
 
 The hypophysis apparently represents a glandular organ, the 
 secretion of which formerly passed into the ventricles, and various 
 hypotheses have been put forward as to its first origin. 
 
 One of the more recent of these theories assumes that it corresponds to the 
 primitive mouth (palceostoma) of the Proto-Vertebrata, which is to a greater 
 or less extent represented by the combined unpaired nasal and pituitary 
 passage of Cyclostomes (see under Olfactory Organ) : the mouth of existing 
 Vertebrates must then be distinguished as a neostoma. 
 
 Both the primary and the secondary fore-brain are situated in 
 the pre-chordal region of the skull, all the other divisions of the 
 brain lying in its chord al portion (comp. p. 67). 
 
 The mid-brain and medulla oblongata undergo fewer modifi- 
 cations than the fore-brain ; only the anterior part of the thin 
 
 - - a Still more anteriorly a third outgrowth or paraphysi*, arising from the 
 secondary fore-brain, has been observed in the embryos of various Vertebrates. 
 
156 
 
 COMPARATIVE ANATOMY 
 
 IT 
 
 roof of the latter (valve of Vieussens) is nervous, and its floor 
 becomes greatly thickened. The greater number of the cerebral 
 nerves arise from the medulla oblongata, so that its physiological 
 importance is very great. The cerebellum 
 may become more or less distinctly sub- 
 divided into lobes. 
 
 In the course of the development of the 
 brain the walls of the cerebral vesicles be- 
 come more and more thickened, so that 
 their cavities undergo a gradual constriction. 
 A series of unpaired ventricles (prosoccele, 
 thalamoccele, mesocaele, metaccele, myeloccele, 
 see p. 153), lying in the longitudinal axis of 
 the brain, as well as paired outgrowths from 
 certain of them, can always be distinguished 
 (Fig. 127). When cerebral hemispheres are 
 developed (as is generally the case), the 
 prosoccele gives rise to paired cavities, ex- 
 tending into them, and known as the lateral 
 ventricles (ventriculus 1 and 2); each of these 
 communicates with the thalamoccele or 
 third ventricle by means of an opening, the 
 foramen of Monro, and may be continued 
 into the corresponding olfactory lobe as a 
 rhinoccelc or olfactory ventricle. Each optic 
 lobe also usually contains an optic ventricle, 
 or optoccelc, communicating with the meso- 
 ccele or aqueduct of Sylvius. There may be a 
 distinct metacoele in the cerebellum opening 
 into the myeloccele or fourth ventricle. 
 
 FIG. 127. DIAGRAM OF 
 THE VENTRICLES OF THE 
 VERTEBRATE BRAIN. 
 
 VH, cerebral hemispheres 
 containing the lateral 
 (1st and 2nd) ventricles 
 (8V) ; ZH, thalamen- 
 cephalon,with the third 
 ventricle (///) ; a thick- 
 ened vascular part of 
 the pia mater (choroid 
 plexus) roofs over the 
 third and fourth ven- 
 tricles ; each lateral 
 ventricle communicates 
 with the third ventricle 
 by a small aperture, the 
 foramen of Monro ( FM) ; 
 MH, mid-brain, which 
 encloses the aqueduct 
 of Sylvius (Aqjf com- 
 municating between the 
 third and fourth vent- 
 ricles ; HH, cerebel- 
 lum ; Nil, medulla ob- 
 longata, enclosing the 
 fourth ventricle (IV) ; 
 Cc, central canal of the 
 spinal chord (7?). 
 
 A so-called fifth ventricle, situated between the 
 corpus callosum and fomix, is found in Mammals, 
 but morphologically it has nothing to do with the 
 ventricles proper, and simply represents a space 
 between the thin internal walls (septa lucida) of 
 the two hemispheres. 
 
 All five cerebral vesicles lie at first in 
 the same horizontal plane, but in the course 
 of development a cerebral flexure takes place, 
 the axis of the vesicles becoming bent down- 
 wards, so that at a certain stage the mesen- 
 cephalon forms the apparent apex of the 
 brain. In Mammals, the parts of the brain 
 become still further folded on one another, so that a parietal, a 
 Varolian, and a cervical bend may be distinguished (Fig. 128) : 
 this process is connected with the further development of the skull 
 and the rapid longitudinal growth of the brain. 
 
THE BRAIN 
 
 157 
 
 Vff ZIf 
 
 Iii Fishes and Amphibians the cerebral flexure later becomes 
 practically obliterated, but it persists more or less markedly in the 
 higher types, more particularly in Mammals. In the latter Class, 
 moreover, the original relation of the parts becomes still further 
 complicated by the large development of the cerebral hemispheres, 
 which grow backwards, and thus gradually come to overlie all the 
 other parts of the brain. This condition of things attains its 
 greatest perfection in Man. Thus instead of 
 the various sections of the brain being situated 
 one behind another, they come to lie eventually 
 more upon one another, the thalamencephalon, 
 mid-brain, cerebellum, and medulla oblongata 
 becoming covered over by the hemispheres. 
 
 Amphioxus. The conical and enlarged anterior 
 end of the spinal cord of the Lancelot contains a 
 widened portion of the central canal which must be 
 looked upon as a ventricle. In the larva, this opens 
 freely on to the exterior dorsally by a neurop&re, 
 which probably represents the last indication of the 
 primitive connection of the central nervous system 
 with the outer skin. It is possible that the anterior VH, prosencephal 
 enlargement of the cord corresponds to the fore- ZH, thalamen 
 
 brain and perhaps also the mid -brain of the 
 Craniata. 
 
 FIG 128. CEREBRAL 
 FLEXURE OF A 
 MAMMAL. 
 
 Cyclostomi. The brain of these forms 
 remains in many respects in an embryonic con- 
 dition : this is particularly the case in the larval 
 Petromyzon or Ammoccete (Fig. 129). In the adult 
 the individual vesicles lie in an almost horizontal 
 direction one behind the other, and the prosen- 
 cephalon consists of a median part and of small 
 paired hemispheres continuous anteriorly with 
 the larger, rounded olfactory lobes. The median 
 portion of the prosoccele is continued trans- 
 versely outwards into each hemisphere, in which 
 it gives rise to a lateral ventricle : this is con- 
 tinued forwards for a short distance into the 
 base of the olfactory lobe, as well as backwards 
 into the hemisphere. The roof (pallium) of 
 the median portion of the ventricle is non- 
 nervous, and consists of a single layer of epithelial cells, which, 
 together with the pia rnater, has been removed in the prepa- 
 ration represented in Fig. 129, A. The mid-brain and medulla 
 oblongata are relatively broad, and the cerebellum is represented 
 by a mere narrow ledge overhanging the fourth ventricle ante- 
 riorly. The roof of the mesocoele is formed mainly bv a layer of 
 epithelial cells, and, like that of the third and fourth ventricles, i 
 
 on; 
 
 thalamence- 
 phalon, with the 
 pituitary body 
 (H) at its base ; 
 MJf, mesenceph- 
 alon, which at SB 
 forms the most 
 projecting por- 
 tion of the brain, 
 representing the 
 so-called " parie- 
 tal bend " ; HH, 
 metencephalon ; 
 NH, myelence- 
 phalon, forming 
 the " cervical 
 bend " (NB) : the 
 "Varolian bend" 
 (BB) arises on 
 the ventral cir- 
 cumference, at 
 the junction be- 
 tween HH and 
 NH; R, spinal 
 cord. 
 
158 
 
 COMPARATIVE ANATOMY 
 
 covered by a thickened and vascular portion of the pia mater or 
 choroid plexus. 
 
 . m\v\v 
 
 ^- YE VI H 
 
 If L.ol 
 
 Hyp 
 
 FIG. 129. BRAIN OF LARVAL LAMPREY. (A, from above ; B, from below ; 
 C, from the side.) 
 
 yjj (Bas.G), cerebral hemispheres, between which, in A, the median portion of the 
 prosencephalon is seen, with the membranous roof removed ; L.ol, olfactory 
 lobe ; ZH, thalamencephalon ; G.p, pineal body ; Hyp, hypophysis ; 8v, 
 saccus vasculosus ; MH, mid-brain ; HH, cerebellum ; Nil, medulla obi on - 
 gata ; Med, spinal cord ; I-X, cranial nerves ; XII, first spinal nerve (hypo- 
 glossal). 
 
 The brain of Myxine shows many special peculiarities : its 
 subdivisions are broader and more closely approximated than in 
 
THE BRAIN 159 
 
 the lamprey, and the thalamencephalon cannot be seen from the 
 dorsal side owing to the larger size of the solid prosencephalon. 
 The mesocoele ends blindly in. front, the third ventricle being 
 almost completely obliterated. The cerebellum is relatively larger 
 than in Petromyzon, and no pallium has been recognised in the 
 prosencephalon of the adult. 
 
 In Petromyzon the pineal apparatus is represented by two 
 vesicles, each connected with the dorsal surface of the thalamen- 
 cephalon (ganglion habenulse) and lying one above the other just 
 beneath the roof of the skull; the integument immediately above 
 these vesicles is pigmentless. The cells on the ventral side of the 
 dorsal vesicle (epiphysis) are arranged radially and contain pig- 
 ment, forming a kind of:' retina, but they show signs of degenera- 
 tion ; the lower vesicle (parietal organ, p. 155) is without pigment. 
 In Myxine there is only a single pigmentless vesicle. 
 
 A saccus vasculosus (comp. pp. 154, 160, ct seq.) is present in 
 connection with the iniundibulum, to which a small pituitary 
 body is attached. 
 
 Elasmobranchii and Holocephali. The brain of these 
 Fishes, like that of Cyclostomes, is in many respects of a specialised 
 form, characteristic of, and confined to, the group, though the par- 
 ticular regions are much more highly developed than in the 
 Cyclostomi. According to its external form two main types can 
 be distinguished. One of these, seen in Spinax, Scymnus, Noti- 
 danus and the Holocephali, is characterised by its very narrow and 
 elongated form, while in the rest of the Elasmobranchii the indi- 
 vidual parts are more closely compressed and approximated together 
 (Fig. 1 30). In almost all Sharks the prosencephalon is relatively 
 much larger than any of the other regions. The olfactory lobes 
 arise from the anterior or antero-laterrd ends of the prosencephalon, 
 and in some Elasmobranchs remain in close connection with the 
 latter : in others in which the olfactory capsules are situated further 
 forwards, they become drawn out into long olfactory tracts each 
 continuous anteriorly with an olfactory bulb from which the olfac- 
 tory nerves arise. 
 
 A division of the prosencephalon into paired halves is hardly 
 indicated at all in Rays, and only slightly so in the commoner Dog- 
 fishes (e.g., Scyllium, Acanthias), in which, however, lateral and 
 olfactory ventricles are present. Only in Scymnus, and to seme 
 extent in the Notidanida?, is there a distinct separation of the 
 pallium into two hemispheres. In Rays there is only a small 
 single prosoccele, the prosencephalon consisting of a practically 
 solid mass, and the olfactory lobes are also solid. 
 
 The thalamencephalon is roofed over by a choroid plexus, and 
 the tube-like epiphysis may reach such a length as to extend beyond 
 the anterior end of the brain for a considerable distance, and pass 
 distally into the roof of the skull : no indication can be seen of a 
 parietal organ. A pair of small lobes the lobi inferiores are 
 
160 
 
 COMPARATIVE ANATOMY 
 
 f.b. b.o. B 
 
 FIG. 130. BRAIN OF Scyllium canicula. (A, dorsal ; B, ventral ; and C, 
 
 lateral view. )] 
 
 f.b, prosencephalon ; b.o, olfactory bulb ; t.o, olfactory tract (very short in 
 Scyllium) ; th, thalamencephalon ; ep, base of pineal body ; if, lobi inferiores ; 
 h.p, hypophysis ; sc, saccus vasculosus ; m.b, mid-brain (optic lobes) ; h. b, 
 cerebellum ; m.d, medulla oblongata ; fr, fourth ventricle ; i-x, cranial 
 nerves (the ventral vagus roots are omitted from Fig. B. The epithelial and 
 vascular roof of the third and fourth ventricles has been removed. 
 
 present on the infundibulum, and an " infundibular gland " or saccus 
 vasculosus is present on the sides and floor of the iofundibulum, 
 which is connected posteriorly with the pituitary body. 
 
THE BRAIX 
 
 161 
 
 The cerebellum is always very large, overlapping the optic lobes 
 and the medulla oblongata to a greater or less extent : it is divided 
 up into several folds lying one behind the other, and usually con- 
 tains a metacoele opening into the fourth ventricle (Figs. 130 and 
 181). In Sharks the medulla oblongata is an elongated cylindrical 
 body, while in Rays it is more compressed and triangular ; at its 
 
 melcL 
 
 FIG. 131. BRAIX OF Cheilo- 
 scyllium. (From Parker and 
 Haswell's Zoology.) 
 
 Viewed from the dorsal side, 
 the roof of the various ven- 
 tricles removed so as to show 
 the relations of the cavities 
 (semi-diagrammatic). 
 
 cer, dilatation from which the 
 metaccele is given off; dia, 
 thalamoccele the reference 
 line points to the opening 
 leading into the infundibu- 
 lum ; iter, aqueduct of Sylvius 
 (mesocffile), into which the 
 optocoeles (opt) open ; meta, 
 myelocrele ; para, lateral ven- 
 tricle ; pros, median part of 
 prosocrele ; rh, rhinoccele. 
 
 TTl.O 
 
 FIG. 132. BRAIN OF 
 
 Lepidosteus. (Dorsal 
 view.) (After Balfour 
 and Parker. ) 
 
 cbl, cerebellum ; c. h, pro- 
 sencephalon ; di, thal- 
 amencephalon ; m.o, 
 medulla oblongata ; 
 off. I, olfactory lobes ; 
 opt. I, optic lobes ; prs, 
 lobes of prosencepha- 
 lon. 
 
 anterior end are two lateral lobes, the corpora restiformw. In 
 electric Rays (p. 146) a pair of lobi electrici arise from the 
 gray matter of the floor of the fourth ventricle, and these en- 
 close a mass of giant nerve-cells. 
 
 Ganoidei. The pallium covering the median prosocoele 
 consists mainly or entirely of epithelial and connective tissue 
 
 M 
 
162 COMPARATIVE ANATOMY 
 
 elements, much as in Cyclostomes. The olfactory lobes are 
 closely applied to the prosencephalon, which gives rise 
 anteriorly to cerebral hemispheres containing lateral ventricles 
 (Fig. 132). 
 
 The well-developed thalamencephalon has a marked ventral 
 flexure and from its roof arises a strong pineal peduncle, the distal 
 end of which extends into a hollow in the cranial roof, but 
 undergoes atrophy, in Amia becoming completely separated off 
 from the brain. 1 Well-developed lobi inferiores are present, 
 and the hypophysis 2 and saccus vasculosus are voluminous: 
 the latter consists largely of glandular tubules which open 
 into the infundibulum, as is also the case in Elasmobranchs 
 (p. 160). 
 
 The large cerebellum gives rise to a valwila cerebelli (comp. 
 Fig. 134) extending forwards into the ventricle of the mid-brain ; 
 the optic lobes are also large. 
 
 The brain of Amia on the whole most nearly approaches that 
 of the Teleostei in structure. 
 
 Teleostei. As is the case in many other Fishes, the brain in 
 most Teleosts by no means fills the cranial cavity, and it is separated 
 from the roof of the skull by a greater or less amount of a fatty 
 and lymph-like fluid. It never attains to so large a relative size 
 as does that of Elasmobranchs. Its form varies greatly, more by 
 far than in any other Vertebrate group, and only the following 
 essential points can be mentioned here. 
 
 The pallium is entirely epithelial in structure (Figs. 133-135), 
 and, moreover, it presents no median involution dividing the 
 anterior part of the prosencephalon into two lateral hemispheres : 
 there is a median prosocoele. The lower part of the prosen- 
 cephalon is made up of large paired basal ganglia (corpora 
 striata) connected together by an anterior commissure. The 
 olfactory lobes are either closely applied to the prosencephalon 
 and contain a small rhinoccele, or they become differentiated into 
 olfactory tract and bulb, as in Elasmobranchs (p. 159). 
 
 The thalamencephalon is very small. The epiphysis (Figs. 
 133, 134) is plainly distinguishable, but it does not pass into 
 the roof of the skull ; an outgrowth arising from the roof of the 
 brain in front of the epiphysis represents the parietal organ, 
 but this becomes constricted off from the brain and disappears 
 during development. 3 Marked lobi inferiores, as well as a 
 
 1 In Polypterus the pineal body gives rise to a peculiar and extremely large 
 epithelial vesicle. In Devonian Ganoids there was a parietal foramen (comp. 
 p. 171). 
 
 2 In Polypterus and Calamoichthys the hypophysis communicates with the 
 mouth-cavity by a hollow duct, even in the adult (comp. p. 155). 
 
 3 A pirietal foramen is, however, often present in the embryo, and persists 
 throughout life in Callicthys. 
 
THE BRAIN 
 
 163 
 
 hypophysis and glandular saccus vasculosus are present, but these 
 vary much in the degree of their development. The saccus 
 
 Lol, 
 
 FIG. 133. BRAIN OF SALMON. (A, dorsal; B, ventral; and C, lateral view. ) 
 
 VH, prosencephalon ; Pall, pallium (in part removed), and EG and Bas.G, basal 
 ganglia (corpora striata) of the prosencephalon ; 'L.ol, olfactory lobe; G.p, 
 pineal body ; Jnf, infundibulum ; Hyp, hypophysis ; Sr, saccus vasculosus ; 
 UL, lobi inferiores ; Tr.opt, optic tract ; Ch, chiasma ; MH, mid-brain ; 
 HH, cerebellum ; NH, medulla oblongata ; Med, spinal cord ; I-X, cranial 
 nerves ; 1 and 2, first and second spinal nerves (the first represents the 
 hypoglossal, XII). 
 
 vasculosus opens by several apertures into the infundibulum, and 
 is surrounded by a blood-sinus. 
 
 M 2 
 
164 
 
 COMPARATIVE ANATOMY 
 
 The mid-brain (Fig. 133) is extremely large relatively, while 
 the thalamencephalon is depressed between it and the prosen- 
 cephalon. 
 
 The extremely well-developed cerebellum is bent upon itself, 
 
 i 
 
 FIG. 134. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART OF 
 THE TELEOSTEAN BRAIN. (Founded on a figure of the Trout's brain by Rabl- 
 Riickhard. ) 
 
 Tco, roof of the optic lobes ; Tf, torus longitudinalis ; Cp, posterior commissure ; 
 Gp, pineal body, with a cavity (Gp l ) in its interior ; Ep, Ep, the epithelium 
 (ependyme), lining the walls of the ventricles ; t, point at which the epithelial 
 roof of the secondary fore-brain (pallium, Pa) becomes continuous with the 
 lining of the anterior wall of the pineal tube ; at/ is seen an outgrowth which 
 represents a rudimentary parietal organ ; V.cm, common ventricle (prosocoele) 
 of the secondary fore-brain ; V.t, third ventricle ; B.ol, N.ol, olfactory lobe 
 and nerve ; C.st, corpus striatum, which lies on either side of the middle 
 line; Ch.n.opt, optic chiasma ; Ci, inferior commissure; Ch, horizontal com- 
 missure ; J, infundibulum ; H, H l , hypophysis ; Sv, saccus vasculosus ; 
 Li, lobi inferiores ; Aq, aqueduct of Sylvius (mesocoele) ; tr, pathetic nerve ; 
 Val, valvula cerebelli. 
 
 overlies the medulla oblongata behind, and is prolonged in front 
 "into the ventricle of the mid-brain as a valvula cerebelli (Fig. 134), 
 as is the case in Ganoids. 
 
 The Teleostean brain is of a specialised type. It has no direct 
 
THE BRAIX 
 
 165 
 
 connection with that of Cyclostomes or Elasmobranchs, but has 
 certainly passed through Ganoid-like stages. 
 
 N 
 
 rh 
 
 pros 
 
 FIG. 135. TRANSVERSE SECTION THROUGH THE FORE PART OF THE TELEOSTEAN 
 
 BRAIN. 
 
 fr, frontal bone, underneath which the pineal tube, Gp, is visible in transverse sec- 
 tion, and below this the pia mater, Pm ; Pa, the pallium, or roof of the sec- 
 ondary fore-brain, formed of a simple epithelial layer ; V.cm, prosocoele ; Ep, 
 ependyme ; T, T, olfactory tracts at the base of the corpora striata (C.st.). 
 
 Dipnoi. Both as regards external and internal structure, 
 certain points of resemblance may be 
 seen between the brain of Dipnoans 
 and that of Elasmobranchs on the 
 one hand and Amphibians on the other. 
 This fact probably indicates that 
 though the Elasmobranchii and Dipnoi 
 have arisen from a common ancestral 
 type, they have become differentiated 
 along different lines. 
 
 The prosencephalon is well de- 
 veloped (Fig. 136) : the thin pallium 
 is mainly nervous, and is involuted 
 along the median longitudinal line so 
 as to completely separate the two 
 hemispheres from one another in 
 Protopterus : in Ceratodus they are 
 united together posteriorly by a narrow 
 commissure. Olfactory lobes arise 
 from the prosencephalon anteriorly, 
 and contain ventricles. 
 
 The thalamencephalon of Pro- 
 topterus presents certain very charac- 
 teristic features, especially as regards 
 its roof. The pineal body has a long 
 stalk, and its distal vesicle perforates 
 the cartilaginous roof of the skull : 
 in the embryo Ceratodus it even 
 reaches as far as the integument. 
 The choroid plexus gives rise to a 
 vesicular organ, and as regards its 
 
 FIG. 136. BRAIN OF Ceratodus 
 fosteri. Dorsal view. (From 
 Parker and Has well's Zoology. ) 
 
 aud, auditory nerve ; cbl, cere- 
 bellum ; fac, facial nerve ; gl, 
 glossopharyngeal ; med, me- 
 dulla oblongata ; mes, mesence- 
 phalon ; oc, oculomotor nerve; 
 opt, optic nerve ; pros, cere- 
 bral hemispheres ; rh, olfac- 
 tory lobes ; vg, vagus nerve. 
 
166 COMPARATIVE ANATOMY 
 
 network of blood-vessels more nearly resembles that of Elas- 
 mobranclis than that of Amphibians. Lobi inferiores are present. 
 Nervous and glandular portions can here also be recognised in the 
 hypophysis. 
 
 The well-marked mid-brain is indistinctly paired in Ceratodus, 
 but is unpaired in Protopterus. 
 
 The cerebellum is relatively much smaller than in Elasmo- 
 branchs and Teleosts, though better developed than in Urodeles : 
 it gives rise to a valvula cerebelli. 
 
 Amphibia. The prosencephalon of Amphibians is distin- 
 guished from that of I)ipnoans by a higher development of the 
 pallium, which, however, even in the latter group, is differ- 
 entiated into an external layer of nerve fibres and an internal 
 cellular layer. The basal ganglia (corpora striata) are less 
 marked, and merely form a more or less prominent thickening 
 of the wall of each hemisphere projecting into the lateral 
 ventricle. 
 
 The Amphibian brain does not, however, lead towards that 
 of Reptiles. Although the prosencephalon is more highly differ- 
 entiated than in lower forms, the thalamencephalon and mesen- 
 cephalon are simpler than in Fishes; and, on the whole, the 
 brain of Amphibians is less complicated than that of any other 
 Vertebrates. 
 
 In Urodeles the individual parts are more elongated and 
 separated from one another than in Anurans, and the thala- 
 mencephalon is therefore more freely exposed. The hemispheres 
 are almost cylindrical and are separated from one another by the 
 pallial fold as far back as the anterior commissure, 1 as in Pro- 
 topterus; while in the Armra (Figs. 137 and 138, A) they are fused 
 together for a short distance anteriorly, where they are continuous 
 with the olfactory lobes. The thalamencephalon and optic lobes 
 are much broader in Anurans than in Urodeles. The cerebellum 
 consists simply of a small transverse fold, and is especially rudi- 
 mentary in Urodeles. 
 
 The infundibulum and hypophysis are well developed, but a 
 saccus vasculosus is no longer so distinct as in Fishes, though traces 
 of it can still be recognised. The epiphysis does not extend 
 beyond the skull in Urodeles, but in Anuraii larvaa it reaches the 
 integument, undergoing reduction later, when the bony skull-roof 
 is formed ; indications of its extracranial portion can, however, 
 sometimes be recognised even in the adult (the " brow-spot " in 
 e.g., Rana temporaria) : thus its intracranial portion does not 
 represent the entire epiphysis. A parietal organ appears to be 
 entirely wanting in all Amphibians with the exception of some 
 few Anura in which traces of it have been described. 2 
 
 In the Gymnophiona the olfactory lobes and hemispheres are 
 
 1 The dorsal part of the anterior commissure has been said to represent a 
 rudimentary corpus callosum (comp. note on p. 174, and Fig. 138, A). 
 
 2 A parietal foramen was, however, present in the Palaeozoic Stegocephala 
 and other extinct Amphibians. 
 
THE BRAIN 
 
 167 
 
 relatively larger than in other Amphibians, and the hemispheres 
 overlap the posterior parts of the brain to a larger extent. 
 A 
 
 
 FIG. 137. BRAIN OF Rana escuhnta. (A, dorsal ; B, ventral; and C, 
 lateral view.) 
 
 VH, cerebral hemispheres ; ZH, thalamencephalon ; MH, mid-brain ; HIT, cere- 
 bellum ; ^H, medulla oblongata ; Med, spinal cord ; I-X, cranial nerves ; 
 la, lateral root of olfactory nerve ; XII (1), ventral root of first spinal nerve 
 (hypoglossal), and 1, its dorsal root ; L.ol, olfactory lobe ; t, space between 
 the two hemispheres ; Tr.oj)t, optic tract ; Jnf, infundibulum ; Hyp, 
 hypophysis. 
 
 Reptiles. The brain of Reptiles reaches a considerably higher 
 stage of development than that of the forms already described, and 
 the individual parts overlie one another to a greater extent, especially 
 in the Again a3 and Ascalabotse. 
 
168 
 
 COMPARATIVE ANATOMY 
 
 The hemispheres are more highly developed, and the cortex is 
 definitely differentiated and contains the characteristic pyramidal 
 cells. In many cases also a distinct hippocampal lobe (Figs. 139, 140) 
 
 1 Lol 
 
 Co,pMH N Illl NH 
 
 Fis. 138. LONGITUDINAL SECTION THROUGH THE BRAIN OF A, Rana, AND B, 
 Hatteria. (A after H. F. Osborn.) 
 
 VH, MH, HH, NH, prosen-, mesen-, meten-, and myelencephalon ; H in (B), 
 hemisphere, which possesses a furrow on its median face, where it is perforated 
 by numerous vascular foramina (S) : this furrow forms the boundary between 
 the hemisphere and olfactory tract, the main root of which is seen at f ; Lol; 
 olfactory lobe ; /, II, IV, origins of the olfactory, optic, and pathetic 
 nerves ; Ep, **, base of epiphysis, which is not shown ; Ch.opt and Ch, optic 
 chiasma ; Lt, lamina terminalis (the reference line should point to the cut 
 edge below Ba and*), Co.a,* anterior commissure ; Ca, Ba, corpus callosum ; 
 F.Mo, Mo, foramen of Monro, above which, in A, is seen the folded choroid 
 plexus ; Cos, superior commissure ; Co.p, posterior commissure ; V 1H and F /r , 
 third and fourth ventricles ; Th. opt and M, optic thalamas ; Lo (in B), aper- 
 ture, and Fu, furrow in the wall of the third ventricle ; Aq, Aq.Syl, aqueduct 
 of Sylvius ; Jnf, infundibulum ; Hyp, hypophysis. 
 
 is present (Hatteria, Chelonia, Crocodilia), and the commissural 
 system between the hemispheres known as the fornix as well as a 
 so-called " corpus callosum " (comp. p. 174) are present in rudiment. 
 
THE BRAIN 
 
 169 
 
 ChTrJnf Hyp 
 
 FIG. 139. BRAIX OF Hatteria punctata. (A, dorsal ; B, ventral ; and C, 
 lateral view.) 
 
 VH, MH, HH, NH, as in Fig. 138 ; Med, spinal cord ; I-XII, cranial nerves ; 
 Lp, process of the hemisphere representing a hippocampal lobe ; N.opt, 
 optic nerve ; Ch, optic chiasma ; Tr, optic tract ; Jiif, infundibulum ; Hyp, 
 hypophysis ; G.p, pineal body, shown in C continuous with the parietal eye 
 (Pa), and only indicated diagrammatically in A ; J? 1 , curved ridge at the 
 base of the optic lobe ; h, small elevation in front of the cer-ebellum. 
 
170 
 
 COMPARATIVE ANATOMY 
 
 ZH- 
 
 HHi 
 
 FIG. 140. BRAIN OF ALLIGATOR. (A, dorsal ; B, ventral ; and C, lateral view.) 
 
 VH, cerebral hemispheres, each of which gives rise postero-laterally to a hippo- 
 campal lobe partially overlying the corresponding optic tract, Tr.opt ; ZH, 
 thalamencephalon ; MH, optic lobes ; HH, cerebellum ; NH, medulla 
 oblongata ; I-XII, cranial nerves ; 1, 2, first and second spinal nerves ; B.ol, 
 olfactory bulb ; Tro, olfactory tract ; G.p, pineal body ; Jnf, infundibulum ; 
 Hyp, hypophysis ; Med, spinal cord. 
 
THE BRAIN 171 
 
 The olfactory lobes may be well marked or entirely invisible 
 externally. In such forms as Anguis, Amphisbsena and Typhlops 
 they are closely applied to the hemispheres, while in others (e.g., 
 Hatteria, Lacerta, Crocodilus) each consists of a well-marked olfac- 
 tory tract, passing anteriorly into an olfactory bulb from which the 
 nerves of smell arise. Olfactory ventricles are usually present. 
 
 The thalamencephalon is always depressed, and is hardly, or 
 not at all, visible from the dorsal side. A distinct hypophysis and 
 
 L ':> . ^ sj 
 
 FIG. 141. LONGITUDINAL SECTION THROUGH THE PARIETAL EYE AND ITS CON- 
 NECTIVE-TISSUE CAPSULE OF Hatteria punctata. (After Baldwin Spencer. ) 
 
 cp, connective-tissue capsule ; r, "lens ;" cr, cavity of the eye, filled with fluid ; 
 r l , retinal portion of the vesicle ; vs, blood-vessels ; C.M, cells in the nerve 
 stalk (s.n.). 
 
 infundibulum as well as an epiphysis are present, and in Lizards 
 the parietal organ retains more or less distinctly, even in the adult, 
 its primitive structure as a median eye. 
 
 This parietal eye (Fig. 141) is situated in the parietal foramen 
 of the skull, and is in close connection with the epiphysis, though 
 in the embryo the nerve which supplies it is seen to arise in- 
 dependently from the brain, in front of the pineal outgrowth. The 
 eye has the form of a vesicle, the dorsal wall of which may become 
 thickened to form a transparent lens-like body, while the rest of 
 
172 COMPARATIVE ANATOMY 
 
 the wall consists of several layers and forms a pigmented retina, 
 with which the more or less rudimentary nerve is continuous. The 
 vesicle is surrounded externally by a connective-tissue capsule, and 
 in many cases the integument and connective-tissue immediately 
 overlying the vesicle is pigmentless and transparent, forming a 
 kind of cornea. Traces of a vitreous body have also been observed. 
 Various degrees of reduction of the different parts as they occur 
 e.g., in Hatteria (Fig. 141), are seen amongst Lizards. (See also p. 
 155). Traces of a parietal eye, with lens and pigment, have also 
 been observed in the embryo of the Viper (Pelias berus). 
 
 In the mid-brain the two well-marked optic lobes may show 
 indications of a further subdivision into four ; from them the optic 
 tracts pass downwards and forwards to the chiasma. The cerebellum 
 is relatively small, except in the Crocodilia (Fig. 140), in which it 
 consists of a thicker median, and two lateral portions, while in 
 other Reptiles, and more particularly in Lizards, it is not much 
 more highly developed than in Amphibians. The medulla ob- 
 longata has a marked ventral flexure. 
 
 Birds. The basal ganglia (corpora striata) of the hemispheres 
 reach a relatively larger size in Birds than in any other Vertebrates, 
 while the differentiation of the cortex and commissures does not 
 show any marked advance on that seen in Reptiles. 
 
 The different parts of the brain overlie one another much 
 more markedly than in any Reptile, and the hemispheres are 
 much larger relatively, covering over the thalamencephalon and part 
 of the mid-brain (Fig. 142). The olfactory lobes are short and 
 conical. The distal, enlarged end of the pineal body extends as 
 far as the dura mater, and the structure of the internal part of the 
 organ resembles that of a tubular gland, penetrated by fibrous tissue 
 and blood-vessels. There is no trace of a parietal organ. 
 
 The cerebellum consists of a well- developed and folded median 
 lobe, and of two lateral portions (flocculi), which vary much both in 
 form and size. Posteriorly it completely covers the fourth ventricle. 
 The two optic lobes are separated from one another and pressed 
 downwards, so as to lie on the sides of the brain in the angle between 
 the hemispheres, cerebellum, and medulla oblongata, and they are 
 connected by a broad commissure. The ventral side of the hind- 
 brain shows a marked flexure, bending upwards to the spinal cord. 
 Mammals. The brain in embryo Mammalia is very similar 
 to that of the Sauropsida, but its later differentiation more 
 particularly that of the pallium gives it a very special character. 
 The cortex becomes much more highly differentiated, and in 
 many Mammals is more or less highly convoluted (Figs. 144, 146), 
 giving rise to gyri and sulci (p. 154). In otheis, again, the surface 
 of the hemispheres remains smooth (Fig. 143), but a subdivision 
 into lobes (frontal, parietal, temporal, &c.) can always be recog- 
 nised to a greater or less extent, and the hemispheres are 
 relatively so large as to cover over the more posterior parts of the 
 brain ; in some of the lower forms, the mid-brain can still be seen 
 
THE BRAIN 
 
 173 
 
 Ntt- 
 
 FIG. 142. BRAIN OF PIGEON. (A, dorsal; B, ventral; and C, lateral view. ) 
 
 VH, cerebral hemispheres ; MH, optic lobes ; Hlf, cerebellum ; NJFf, medulla 
 oblongata; Med, spinal cord ; I- XII, cranial nerves ; 1, 2, first and second 
 spinal nerves ; L.ol^ olfactory lobes ; Tr.opt, optic tract ; Jnf, iiifundibulum ; 
 Hyp, hypophysis. 
 
 from above (Fig. 143) while in the higher types (Primates) even 
 part of the cerebellum is hidden (Figs. 145, 146). 
 
 The commissures between the hemispheres (corpus callosum 
 and fornix, Fig. 145) are also much more highly developed than 
 in the Sauropsida. The corpus callosum or pallial commissure, 
 though small in the lower Mammalia (e.g., Monotremes and 
 
174 
 
 COMPARATIVE ANATOMY 
 
 Marsupials), 1 is usually a large and important structure ; its 
 relative size is in inverse proportion to that of the anterior 
 commissure. In addition to the anterior and posterior commis- 
 sures, a middle commissure is definitely differentiated from the 
 
 f.b. 
 
 f-p.- 
 
 m iv |V [yiii] x xi 
 p.v. vi vii ix x 
 
 FIG. 143. BRAIN OF RABBIT. (A, dorsal ; B, ventral ; and C, lateral view.) 
 
 f. b. , cerebral hemispheres ; m. b. , optic lobes ; h. b. , cerebellum ; c. b'. , superior 
 vermis, and c. b". , lateral lobe of cerebellum ; md. , medulla oblongata ; ep. , 
 pineal body ; h. p., hypophysis ; pv., pons Varolii ; cr. , crura cerebri ; f,p., 
 pallial fissure ; b.o., olfactory bulb ; i-xii, cerebral nerves. 
 
 base of the brain as a distinct structure connecting the two optic 
 thalami. 
 
 In correspondence with the division of the hemispheres into 
 lobes, there is a marked differentiation of the lateral ventricles, 
 
 ' l Recent researches indicate that a true corpus callosum is present only in 
 the Placentalia, and that the commissure which is usually supposed to represent 
 it in lower types may be more correctly described as the hippocampal commissure. 
 
THE BRAIN 
 
 175 
 
 so that an anterior, a -posterior, and an inferior cornu can be dis- 
 tinguished in each ; the inferior cornu extends into what corresponds 
 to the hippocampal lobe of Reptiles (p. 168), and an eminence on 
 its floor, known as the hippocampus major , is much more marked 
 than in lower forms. The olfactory lobes, in which an olfactory 
 
 r TZT 
 
 Rol 
 
 FIG. 144. BRAIN OF DOG (POINTER). (A, dorsal; B, ventral; and C, 
 lateral view.) 
 
 VH, cerebral hemispheres ; MH, optic lobes ; HH, cerebellum , Wu, superior 
 vermis ; HH 1 , lateral lobe of cerebellum ; NH, medulla oblongata ; Mtd, 
 spinal cord; Hyp, hypophysis; Po, pons Varolii : Cr.ce, crura cerebri ; 
 Fi.p, pallial fissure ; B.oi, olfactory bulb ; I-XII, cranial nerves. 
 
 tract and bulb can be distinguished, usually extend forwards freely 
 from the base of the prosencephalon and each may (e.g., Horse) 
 contain a prolongation of the lateral ventricle ; but in some cases 
 (e.g., numerous aquatic forms and Primates) they are completely 
 covered by the frontal lobes. 
 
176 
 
 COMPARATIVE ANATOMY 
 
 The pineal body is displaced downwards by the hemispheres 
 and lies against the anterior part of the mid-brain, not reaching 
 to the roof of the skull. Its bifurcated peduncle connects it 
 
 -ffff 
 
 FIG. 145. HUMAN BRAIN. (Median longitudinal vertical section. ) 
 
 (Mainly after Reichert. ) 
 
 VH, cerebrum ; To, optic thalamus (thalamencephalon), with the middle commis- 
 sure (Cm)-, Z, pineal body; T, infundibulum ; H, pituitary body; MH, 
 corpora bigemina, with the aqueduct of Sylvius (Aq), anterior to which is 
 seen the posterior commissure (Cp) ; HH, cerebellum ; NH, medulla oblongata, 
 with the pons Varolii (P); R, spinal cord; B, corpus callosum ; G, fornix, 
 which extends antero- vent/rally to the lamina terminalis (Col), in the upper 
 part of which is seen the anterior commissure (Ca), and between the latter 
 and the optic thalami (To) the foramen of Monro (FM) ; Teh, tela choroidea ; 
 /, olfactory nerve ; //, optic nerve. 
 
 FIG. 146. CONVOLUTIONS or THE HUMAN BRAIN. (After A. Ecker.) 
 Lf frontal lobe ; Lp, parietal lobe ; Lo, occipital lobe ; T, temporal lobe ; a, b, c, 
 ' superior, middle, and inferior frontal gyri ; X, /3, anterior and posterior 
 central convolutions, separated from one another by the fissure of Rolando 
 (R) ; cm, the calloso-marginal sulcus on the dorsal surface ; P, P 1 , superior and 
 inferior parietal gyri separated from one another by the interparietal fissure 
 (/) Po, parietal-occipital fissure ; FS, Sylvian fissure ; 1 to 3, superior, 
 middle, and inferior temporal convolutions ; HH, cerebellum ; NH, medulla 
 oblongata ; R, spinal cord. 
 
 with the roof of the thalamencephalon and contains nervous 
 substance ; its distal end has the form of a rounded or oval sac, 
 consisting of compact epithelial tissue and containing concre- 
 tions. No indication of a parietal organ can be recognised. 
 
PERIPHERAL NERVOUS SYSTEM 177 
 
 Traces of the saccus vasculosus and lobi inferiores still occur, even in Man, 
 in connection with the iiifimdibulum. 
 
 The mid-brain is of smaller relative size than in other 
 Vertebrates. A transverse furrow across the solid optic lobes sub- 
 divides them into an anterior larger and a posterior smaller pair of 
 lobes (comp. p. 172). 
 
 The division of the cerebellum into a median and two lateral 
 portions, already indicated in Reptiles, but much more plainly 
 marked in Birds, is carried to a still further extent in Mammals. 
 The median portion gives rise to the so-called superior vermis 
 while the lateral parts form the lateral lobes and flocculi (Figs. 143, 
 144). The two lateral lobes are connected by a large commissure, 
 the pons Varolii (Figs. 1 43-145) : this extends round the medulla 
 oblongata ventrally, and is more largely developed the higher 
 we pass in the Mammalian series. Other bands of nerve-fibres 
 connecting the cerebellum with other parts of the brain are 
 spoken of as anterior, middle, and posterior peduncles of the 
 cerebellum. 
 
 The brain in Cretaceous Birds (e.g., Hesperornis) and in Tertiary 
 Mammals (e.g., Dinoceras, Triceratops) was much less highly developed,, 
 and he hemispheres relatively much smaller, than in existing forms. 
 
 II. PERIPHERAL NERVOUS SYSTEM. 
 
 Two principal groups of peripheral nerves may be distinguished f 
 viz., spinal and cerebral, that is, those which arise from the 
 spinal cord and brain respectively : by their means a physiological 
 connection is established between the periphery of the body and 
 the central nervous system both in centripetal and centrifugal 
 directions. The spinal nerves retain the more primitive and 
 simple relations, and all show a similar arrangement along both 
 dorsal and ventral regions of the spinal cord, so that each segment 
 of the trunk possesses a dorsal and a ventral pair. The former 
 consists of sensory, the latter of motor fibres (Fig. 147). 
 
 Each dorsal or sensory nerve has a ganglion in connection 
 with it, while in the ventral nerves a ganglion is wanting, at any 
 rate in the adult. The ventral nerves arise as direct outgrowths 
 from the spinal cord, while the dorsal nerves first appear as 
 outgrowths from their ganglia, coming into connection with the 
 cord secondarily. The ganglia themselves are developed from a 
 neural ridge of epiblast cells lying close to the junction of the 
 medullary cord (p. 149) and outer epiblast. On the distal side 
 of each ganglion, both nerve-roots almost always become bound 
 up in a common sheath, though many facts seem to indicate that 
 in the ancestors of existing Vertebrates the dorsal and ventral 
 
 N 
 
178 
 
 COMPARATIVE ANATOMY 
 
 FIG. 147. DIAGRAM ILLUSTRATING THE ORIGIN, COURSE, AND TERMINATION OF 
 THE MOTOR AND SENSORY FIBRES OF THE SPINAL NERVES, AS WELL AS THE 
 RELATIONS OF THE SENSORY COLLATERAL FIBRES TO THE POINTS OF ORIGIN 
 OF THE "V ENTRAL ROOTS. (After M. V. Lenhossek. ) 
 
 The spinal cord is shown as if transparent. The fibres of the ventral roots 
 arise from the cells of the motor ventral cornua of the gray matter (a) and 
 end in fine branches on the striated muscle fibres (c). The spinal ganglion 
 (d) is shown relatively much larger than in reality, and in it only a single 
 unipolar nerve-cell is represented : the centripetal fibre of the latter is seen 
 entering the dorsal root, and at e bifurcates in the spinal cord into an anterior 
 (/) and a posterior (g) branch, each of which ends freely in the gray substance, 
 first giving off numerous collateral fibres (h). The centrifugal fibre of the cell 
 in the spinal ganglion forms a peripheral sensory fibre extending to the skin, 
 where part of it is shown ending in fine branches in the epidermis (i), another 
 part forming a coil in connection with a tactile corpuscle (k). 
 
 roots remained distinct, as, in fact, is still the case in Amphioxus 
 and Petromyzon. 
 
 The common nerve-trunk formed by the junction of the two 
 
SPINAL NERVES 179 
 
 roots divides up again into a dorsal, a ventral, and a visceral 
 branch. The first of these goes to the muscles and skin of the 
 back, the second supplies the lateral and ventral portions of the 
 body-wall, while the intestinal branch comes into connection with 
 the sympathetic (p. 188). 
 
 1. SPINAL NERVES. 
 
 As a general rule, each corresponding pair of dorsal and ventral 
 roots lies in the same transverse plane : an exception to this is 
 seen, however, in Amphioxus, 1 Cyclostomes, Elasmobranchs, and 
 Dipnoans, in which the mesoblastic somites of the right and left 
 side are arranged alternately, and thus the points of exit of the 
 nerve-roots also alternate right and left, or each ventral pair 
 alternates with a dorsal pair. In Ganoids also, lateral displace- 
 ments of the nerve-roots are met with. 
 
 In Fishes the greatest variations are seen as regards the mode 
 of exit of the nerves (which pass through the intercalary pieces 
 of the vertebral column, through the arches, or between them) ; 
 but from the Amphibia onwards the nerves always make their 
 exit on each side between the arches, through the intervertebral 
 foramina. 
 
 In their primitive undifferentiated condition the spinal nerves 
 have a strictly metameric arrangement, and are equally developed 
 in all regions of the body. As already pointed out in the section on 
 the spinal cord, this condition becomes modified by the development 
 of the appendages, so that a number of spinal nerves unite together 
 to form plexuses, which, according to their position, are spoken of 
 as cervical, brachial, lumbar, and sacral (Fig. 121). The number 
 of nerves composing these corresponds to the number of body- 
 segments taking part in the formation of the appendages, and 
 their relative size is usually directly proportional to the develop- 
 ment of the latter. 
 
 In contrast to Fishes, the great variation in the plexuses of 
 which renders it impossible to reduce them to a common plan, we 
 find from the Amphibia onwards a typical grouping of the branches 
 of the brachial plexus, from which numerous nerves arise supplying 
 the shoulder and fore-limb dorsally and ventrally (e.g., thoracic, 
 subscapular, axillary, radial, musculo-cutancous, and ulnar}. The 
 lumbo-sacral plexus shows in general, and more particularly in 
 Mammals, much greater variations than does the brachial plexus. 
 The nerves arising from it are also arranged in a dorsal and a 
 ventral series, the larger ones being spoken of as the obturator. 
 
 1 In Amphioxus both the dorsal and ventral nerves innervate muscles, and 
 it appears that in many of the Craniata also the dorsal roots are not purely 
 sensory. 
 
 N 2 
 
180 COMPARATIVE ANATOMY 
 
 crural, sciatic, and pudendic. The sciatic divides up in the hind- 
 limb into a tibial and a fibular nerve. 1 
 
 2 CEREBRAL NERVES. 
 
 The following twelve pairs of cerebral nerves can be distin- 
 guished, and of these the eleventh pair are plainly differentiated 
 only in the Amniota, and the twelfth are represented by the first 
 spinal nerves in certain Fishes and in all Amphibians : 
 
 I. Olfactory. 
 
 II. Optic. 
 
 III. Oculomotor. 
 
 IV. Pathetic or trochlear. 
 V. Trigeminal. 
 
 VI. Abducent. 
 
 VII. Facial. 
 
 VIII. Auditory. 
 
 IX. Glossopharyngeal. 
 
 X. Vagus or piieumogastric. 
 
 XI. Spinal accessory. 
 
 XII. Hypoglossal. 
 
 In their mode of early development the cerebral nerves resemble- 
 the spinal nerves in many respects (p. 177), and a gradual tran- 
 sition between the two groups is indicated in the lower Vetebrata. 
 Certain of them, like the motor spinal nerves, arise as direct 
 ventral outgrowths from the embryonic brain (III, VI, XII, and 1 
 probably IV 2 ). Others, again (V and VII in part, VIII, IX, 
 and X), arise dorsally, primarily in connection with their indi- 
 vidual ganglia and becoming actually connected with the brain 
 secondarily : these must therefore, so far as they consist of sensory, 
 centripetal elements, be looked upon as homodynamous with the 
 dorsal roots of the spinal nerves. But it must be borne in mind 
 that all these nerves, with the exception of the olfactory, optic, 
 and auditory, are of a mixed character, containing motor as well 
 as sensory fibres ; and a further difference between them and the 
 dorsal roots of the spinal nerves (comp. note on p. 179) is seen in 
 the shifting of their origin to the ventral side of the brain during 
 development. 
 
 A study of development shows that portions of the epiblast lying 
 peripherally to the brain take part in the formation of the ganglia of the 
 trigeminal, facial, auditory, and vagus nerves, and that each definitive 
 
 1 In animals in which the extremities have disappeared, all traces of the 
 corresponding plexuses have also usually vanished : Snakes, however, still retain 
 remnants of them. 
 
 2 The fourth nerve is peculiar in appearing from the dorsal surface of the 
 brain, but this is probably a secondary condition p. 184). 
 
CEREBRAL NERVES 
 
 181 
 
 ganglion consists of a primary " spiiial" ganglion and of a more peripheral 
 Lntvi'id ganglion in connection with the nerve, from which latter an epi- 
 branchial ganglion arises from the epiblast dorsally to the region of the gill- 
 clefts, and takes part in the formation of the terminal branches of the nerve. 
 The presence of an epibranchial ganglion on the trigeminal may indicate 
 the former presence of a gill-cleft in this region. 
 
 It must be remembered that the head is primitively composed 
 of a series of metameres (p. 66), and it is therefore important to 
 ascertain, as far as is possible in the present state of our knowledge, 
 to which individual metameres the different cranial nerves belong. 
 The olfactory and optic nerves present certain peculiarities which 
 bring them under another category, and they will be treated of 
 later in connection with the corresponding sensory organs. 
 
 The following general summary gives a scheme of the prob- 
 able primitive relations of the head-segments and cerebral 
 nerves, founded mainly on the conditions existing in Elasmo- 
 branch embryos. 
 
 TABLE SHOWING THE SEGMENTAL ARRANGEMENT or THE CEREBRAL NERVES, 
 WITH THEIR RELATION TO THE METAMERES OF THE HEAD. 
 
 Ventral branch. 
 
 Dorsal branch. 
 
 \*t Metamere (superior, in- 
 ferior, and anterior rec- 
 tus, and inferior oblique 
 muscle). 1 
 
 '2nd Metamere (superior 
 oblique). 1 
 
 Oculomotor (///). 
 
 Trochlear (IV). 
 
 3rd Metamere 
 rectus). 1 
 
 (posterior Abducent (VI). 
 
 4th Metamere (muscles 
 which are early aborted). 
 
 oth Metamere (muscles 
 which are early aborted). 
 
 Qth and 1th Metamere* (part 
 of the most anterior 
 region of the large trunk- i 
 muscles). 
 
 8th and 9th Metameres (an- 
 terior part of trunk- 
 muscles). 
 
 Wanting. 
 
 Wanting. 
 
 Appears to be 
 wanting. 
 
 Ventral roots of the 
 hypoglossal. 
 
 Ramus ophthalmicus pro- 
 fundus of the trigeminal 
 (V), together with the 
 ciliary ganglion. 
 
 Trigeminal (with its gang- 
 lion, minus the ramus 
 ophthalmicus profun- 
 dus). 
 
 1 Facial ( VII), and audi- 
 tory ( VIII), with their 
 ganglia. 
 
 1 Glossopharyngeal 
 with its ganglion. 
 
 Vagus (X), with its gang- 
 lia. 
 
 Vestigial dorsal roots of 
 the hypoglossal (XII), 
 usually only present in 
 the embryo. 
 
 Figures 148 and 149 illustrate the distribution of the cerebral 
 nerves in adult aquatic and terrestrial Vertebrates respectively (comp. 
 
 1 It is possible, however, that these eye-muscles belong, not to the somites, 
 as stated on pp. 133 and 143, but to the visceral muscles. 
 
182 
 
 COMPARATIVE ANATOMY 
 
 I * 
 
 iiil 
 
CEREBRAL NERVES 
 
 183 
 
 H ij 
 
 rHsl2ijftii5! 
 
184 COMPARATIVE ANATOMY 
 
 also Fig. 121). The ganglia belonging to the cerebro-spinal system 
 are shown in both figures, those belonging to the sympathetic in 
 Fig. 149 only. 
 
 Nerves of the Eye-muscles. The oculomotor (III) 
 trochlear or pathetic (IV) and abducent (VI) nerves (Figs. 
 148 and 149) supply the muscles which move the bulb of the eye 
 as shown in the table on p. 181. The oculomotor arises from 
 the base of the mid-brain, and comes into secondary connection with 
 an oculomotor or ciliary ganglion which primarily belongs to the 
 sympathetic system. 
 
 The trochlear nerve, although actually arising in the interior 
 of the ventral part of the mid-brain, appears externally on the 
 dorsal side of the anterior margin of the hind-brain (valve of 
 Vieussens p. 156). Primitively it contains sensory as well as motor 
 fibres, and these in Fishes and Amphibians supply the connective- 
 tissue of the eye and the endocranium. 
 
 The abducent nerve, which arises far back on the floor of 
 the medulla oblongata, also probably contains mixed fibres in the 
 Anamnia. In the Anura it becomes closely connected within the 
 skull with the Gasserian ganglion of the trigeminal. 
 
 Trigeminal Nerve. This is one of the largest of the cerebral 
 nerves. It arises from the ventro-lateral region of the anterior 
 part of the medulla oblongata by a large lateral sensory and a 
 small ventral motor root, has a large intra- or extra-cranial 
 Gasserian ganglion at the origin of the former and then, in 
 Fishes (Fig. 148), divides into two main branches, an ophthalmic 
 (including a superficial and a deep or prqfundus portion), and a max- 
 illo-mandibular : in most terrestrial forms (Fig. 149) the maxillary 
 and mandibular nerves arise separately. From the presence of these 
 three characteristic branches, often known as the first, second, and 
 third divisions of the trigeminal, its name is derived. It passes 
 out from the skull sometimes through a single aperture, and some- 
 times by two or even three distinct ones. 
 
 The superficial branch of the first division is usually distinct in 
 Fishes and Dipnoans and probably also in Urodeles, and passes 
 dorsally over the eye-ball, the deep branch passing below the supe- 
 rior and anterior recti and superior oblique muscles. In other 
 Fishes and in higher forms the two branches appear to be united. 
 It supplies the integument of the forehead and snout as well as 
 the eye-ball, eye-lids and conjunctiva, branches apparently going to 
 the lachrymal glands when present : it is entirely sensory. A con- 
 section of the profundus with the ciliary ganglion arises second- 
 arily. 
 
 The second division of the trigeminal, which is also a sensory 
 nerve and with which a sphenopalatine ganglion derived from the 
 sympathetic is connected, extends first along the floor of the 
 

 CEREBRAL NERVES 185 
 
 orbit, supplying the lachrymal and Harderian glands, when present, 
 as well as the roof of the mouth ; it then passes to the upper jaw, 
 supplying the teeth; and finally, as the infraorbital branch, per- 
 forates the skull to reach the integument in the region of the 
 upper jaw, snout, and upper lip. 
 
 The third division of the trigeminal is of a mixed nature; it 
 supplies on the one hand the masticatory muscles and several 
 muscles on the floor of the mouth, and also gives rise, from 
 Amphibians onwards, to the great sensory nerve of the tongue 
 (lingual or gustatory nerve) ; while another branch, passing 
 through the inferior dental canal, supplies the teeth of the lower 
 jaw, and then gives off one or more branches to the integument of 
 the latter and of the lower lip. Two ganglia, the submaxillary and 
 the otic (Fig. 149), derived from the sympathetic, are connected 
 with its mandibular division (sensory portion). 
 
 Facial nerve. This, which is also a mixed nerve, originally 
 possesses two distinct ganglia in connection with its sensory and 
 mixed portion (Fig. 148) : these can be recognised up to Urodeles, 
 but in the course of development one of them gradually comes into 
 connection with the ganglion of the trigeminal, and in Anura is 
 indistinguishable from it. The other known as the geniculate 
 ganglion is retained in all Vertebrates, in connection with its 
 mixed root (Fig. 149). 
 
 The facial nerve consists primarily (in aquatic Vertebrates) of 
 the following main branches (Fig. 148) : 
 
 I. A system of sensory branches for the supply of the integu- 
 mentary sense-organs of the head (p. 190), 1 as follows : (a) a super- 
 ficial ophthalmic, running- parallel to and usually accompanying the 
 corresponding branch of the trigeminal; (b) a buccal, which gives 
 off an otic branch ; and (c) an external mandibular ( = part of the 
 hyomandibular, see below). 
 
 II. A sensory (a) palatine, anastomosing with the maxillary 
 branch of the trigeminal, and (b) internal mandibular or chorda 
 tympani. 
 
 III. A main trunk, largely motor ( = hyomandibular less the 
 elements which give rise to the sensory external mandibular), 
 which passes behind the spiracle, all the other branches passing 
 in front of it. 
 
 In adult terrestrial Vertebrates (Caducibranchiate Urodeles. 
 Anura, and Amniota) the integumentary sense-organs become re- 
 duced, and the corresponding branches of the facial nerve undergo 
 corresponding reduction (Fig. 149); the parts of this nerve which per- 
 sist are the pharyngeal section (palatine and chorda tympani) and 
 
 1 These branches, together with the lateral line branches of the glosso- 
 pharyngeal and vagus (p. 187) appear to form an independent and distinct 
 system of lateral line nerves, having a common internal origin in the brain, for the 
 innervation of the special sensory organs of the integument in Fishes, Dipiioans 
 and Amphibians. The auditory nerve arises from the same centre. 
 
186 COMPARATIVE ANATOMY 
 
 the main trunk (hyomandibular less its lateral line elements). The 
 latter is connected with the glossopharyngeal by the anastomosis 
 of Jacobson, and is distributed, as its name implies, to the region 
 of the first and second visceral arches : thus in Fishes it goes 
 to the parts around the spiracle and to the muscles of the oper- 
 culum and branchiostegal membrane. A small remnant of this 
 branch in the higher Vertebrates supplies the stylohyoid muscle and 
 the posterior belly of the digastric and the stapedius. 
 
 In Mammals the facial is mainly a motor nerve. It is chiefly 
 important in supplying the facial muscles, as well as the platysma 
 myoides, which has the closest relation to them (p. 136). The 
 more highly the facial muscles are differentiated (e.g. Primates, 
 especially Homo), the more complicated are the networks formed 
 by the facial nerve. 
 
 Auditory Nerve. This large nerve arises in close connection 
 with the facial, and corresponds to a sensory portion of the latter 
 nerve ; 1 it possesses a ganglion (Figs. 148 and 149). Soon after its 
 origin from the brain it divides into a cochlear and a vestibular 
 branch. The former passes to the lagena or cochlea, while the 
 latter supplies the rest of the auditory labyrinth. 
 
 Vagus group. This group includes the glossopharyngeal, 
 vagus, and spinal accessory, which stand in the closest relation 
 to one another, and are more nearly comparable to the spinal 
 nerves than are the cerebral nerves already described. It consists of 
 both sensory and motor fibres, the former being connected with 
 ganglia (the jugular and pctrosal). The distribution of these 
 nerves differs from that of the other cerebral nerves in not being 
 limited to the head. 
 
 Thus the vagus supplies not only the pharynx, tongue, and 
 respiratory organs, but also sends branches to the heart, larynx, 
 and a considerable portion of the digestive tract, as well as to 
 integumentary sense-organs of the trunk in Fishes. 
 
 The spinal accessory nerve appears for the first time in the 
 Amniota, and will be dealt with after the vagus and glossopharyn- 
 geal have been described (p. 187). 
 
 The origin of both glossopharyngeal and vagus by numerous 
 roots in Fishes (Fig. 148) indicates that these nerves correspond to 
 a number of spinal nerves, and this comparison is further justified 
 by the fact that they give off branches in the region of the pharynx 
 and visceral arches, in which a metameric arrangement can be 
 recognised. 
 
 In many Fishes and in Dipnoans two or three nerves make their exit from 
 the skull ventrally to the root of the vagus (Fig. 148) : these " spino-occipital " 
 
 1 On the supposition that the auditory organ corresponds to a modified 
 integumentary sense-organ, the auditory nerve would belong to the lateral line 
 system of nerves (see note on p. 185). 
 
CEREBRAL NERVES 187 
 
 or intracranial spinal nerves, which have been described as ' ventral roots " 
 of the vagus (see p. 143), have nothing to do with this nerve, and perhaps 
 correspond to a part of the hypoglossal of higher Vertebrates. 
 
 In Fishes and perennibranchiate Amphibians the glosso- 
 pharyngeal leaves the skull through a special foramen, and not along 
 with the vagus, a lateral line branch l of which arises separately 
 from and anteriorly to the rest of nerve, dorsally to the glosso- 
 pharyngeal and near the origin of the sensory part of the facial 
 (Fig. 148). This lateral nerve, which may divide into two or even 
 three branches, extends along the side of the body to the tail, 
 either directly beneath the skin, or close to the vertebral column 
 (e.g. Elasniobranchii, Dipnoi), and supplies integumentary sense 
 organs. 
 
 In Protopterus the vagus also gives off superficial branches which extend 
 along the dorsal, lateral and ventral regions of the body close to the skin. 
 In certain Teleosts (Anacanthini) dorsal and ventral superficial nerves are 
 also present, which have sometimes been described as cutaneous branches of 
 the trigeminal. These require further investigation : they appear to belong 
 mainly to the facial, and from their origin and distribution correspond pre- 
 cisely to the " ramus dorsalis recurrens " of Siluroids. The vagus invariably 
 takes part in their formation, and sometimes also the glossopharyngeal and 
 even the first spinal nerves. 
 
 In tracing the development of the lateral nerves, the nervous elements 
 are seen to be so closely united with the thickened epidermis in the region 
 of the lateral line that it is impossible to say whether the nerve arises in sitii 
 or not ; and this is also the case as regards all nerves (VII. , IX. , X. ) supply- 
 ing integumentary sense organs in the Anamnia. 
 
 In branchiate Vertebrates, the glossopharyngeal gives off a 
 pharyiigeal branch and forks over the first branchial cleft, while the 
 vagus gives rise to branchial branches which are similarly related to 
 the following clefts (Fig. 148) : these branchial nerves supply the 
 muscles and mucous membrane of the branchial apparatus. In 
 Chimsera each of the three branchial nerves arises independently 
 from the brain. It will be remembered that the facial nerve has 
 similar relations to the spiracuiar cleft (p. 185). Both glossopharyn- 
 geal and vagus contain mixed fibres, and become connected in various 
 ways with the trigeminal and facial. In correspondence with the 
 reduction of the gills in higher forms, the branchial branches of the 
 vagus can no longer be recognised, and the glossopharyngeal passes 
 into the tongue as the nerve of taste, giving off also a pharyngeal 
 branch (Fig. 149). This condition is first indicated in Dipnoi and 
 Amphibia. 
 
 The spinal accessory nerve first appears distinctly in Reptiles. 
 It arises some distance back along the cervical portion of the 
 spinal cord, in the region from which the fourth to fifth cervical 
 nerves come off; from this point it passes forwards as a collector, 
 taking up fibres from the cervical nerves as it goes. It extends 
 along the side of the medulla oblongata into the cranial cavity, and 
 
 1 The glossopharyngeal also possesses a lateral line branch in many Fishes. 
 
188 COMPARATIVE ANATOMY 
 
 leaves the skull through the same foramen as the vagus, to which 
 it gives off motor elements. It supplies certain of the muscles 
 related to the pectoral arch, e.g. the sternocleidomastoid and the 
 trapezius. 
 
 Hypoglossal. The hypoglossal corresponds to one or several 
 of the anterior spinal nerves, and its transformation into a cerebral 
 nerve can be traced in passing through the Vertebrate series. 
 In some Fishes and all Amphibia it does not pass through the 
 cranial wall and is a true spinal nerve ; and in most Fishes and in the 
 Dipnoi, its inclusion within the skull can be seen to be due to a 
 gradual assimilation of the anterior part of the vertebral column with 
 the skull (comp. p. 45). In addition to its numerous ventral-roots 
 one or more dorsal, ganglionated roots have been observed in the 
 embryos of various Vertebrates (Figs. 148 and 149). Two dorsal 
 roots, each with a ganglion, persist in Protopterus, and the same is 
 apparently true as regards Polypterus and certain Elasmobranchs : 
 even amongst Mammals, these roots can exceptionally be recog- 
 nised subsequently to the embryonic period. 1 
 
 In Fishes (Fig. 148) the hypoglossal, like the next following 
 spinal nerves, sends branches to the muscles of the body, the 
 floor of the mouth, and skin of the back, as well as being connected 
 with the brachial plexus. In higher Vertebrates (Fig. 149) it 
 supplies the intrinsic and extrinsic muscles of the tongue. 
 These lingual branches are most marked in Mammals, in which 
 the tongue reaches its highest development. Elements of the 
 cervical spinal nerves also run along with the hypoglossal, and 
 give rise to the so-called ramus descendens with which further 
 cervical nerves are associated ; and from the " ansa hypoglossi " 
 thus formed, branches pass to the sterno-hyoid, sternothyroid, 
 omohyoid, and thyrohyoid muscles. 
 
 Sympathetic. 
 
 The sympathetic system arises in close connection with the 
 spinal system, with which it remains throughout life in close 
 connection by means of rami communicantes. It is distributed 
 mainly to the intestinal tract (in the widest sense), the vascular 
 system, and the glandular organs of the body. The sympathetic 
 ganglia, like those of the spinal nerves, show originally a segmental 
 arrangement. They usually become united together later by 
 longitudinal commissures and thus give rise to a chain-like 
 paired sympathetic cord lying on either side of the vertebral 
 column. From its ganglia nerves pass off to the above-mentioned 
 
 1 The dorsal root of the first spinal nerve may be reduced or wanting in 
 Mammals even in Man, so that here the modification of the primary 
 character of the nerves is not limited to those within the skull. 
 
SENSORY ORGANS 
 
 systems of organs, forming numerous plexuses. Peripheral ganglia 
 are also present in the viscera. 
 
 The sympathetic extends not only along the vertebral column, 
 but passes anteriorly into the skull, where it comes into relations 
 with a series of the cerebral nerves (comp. pp. 184, 185 and Fig. 
 149) similar to those which it forms further back with the spinal 
 nerves. 
 
 The original segmental character frequently disappears later on 
 and this is especially the case in those regions where marked 
 modifications of the earlier metameric arrangement of the body 
 have taken place viz., in the neck and certain regions of the 
 trunk, especially towards the tail : thus there are never more than 
 three cervical ganglia in Mammals. 
 
 A sympathetic is not known to exist in Amphioxus, and in 
 Petromyzon it appears to be rudimentary. In Fishes proper, it is 
 more highly differentiated, especially in the head region, while in 
 Dipnoans it has not been observed. In Amphibians the 
 sympathetic is well developed, especially in the higher forms 
 (Fig. 121). In the Myctodera it extends anteriorly to the vagus 
 ganglion and posteriorly through the trunk and haemal canal 
 almost to the apex of the tail, as is the case also in Teleostei. 
 
 In the Sauropsida the cervical portion of the sympathetic is 
 usually double, one part running within the vertebrarterial canal 
 alongside the vertebral artery. In all other Vertebrates the whole 
 cord lies along the ventral and lateral region of the vertebral 
 column : it is generally situated close to the latter, and overlies 
 the vertebral ends of the ribs. 
 
 III. SENSORY ORGANS. 
 
 The specific elements of the sensory organs originate, like the 
 nervous system in general, from the epiblast ; the peripheral ter- 
 minations of the sensory nerves are thus always to be found in 
 relation with cells of ectodermic origin, which become secondarily 
 connected by means of nerve-fibres with the central nervous 
 system. 
 
 The sensory apparatus was primarily situated on a level with 
 the epidermis and served to receive sensory impressions of but 
 slightly specialised kinds ; but in the course of phylogeny parts of it 
 passed inwards beneath the epidermis, and certain of these became 
 differentiated into organs of a higher physiological order, viz. r 
 those connected with smell, sight, hearing, and taste. These are 
 situated in the head, and except the last mentioned, become 
 enclosed in definite sense-capsules (p. 68) ; they must be dis- 
 tinguished from the simpler integumentary sense-organs, which are 
 concerned with the senses of touch, temperature, &c. 
 
 In many, and more especially in the higher sensory organs, 
 
190 COMPARATIVE ANATOMY 
 
 supporting or isolating cells can be recognised in addition to the 
 sensory cells proper ; both kinds, however, being ectodermic. The 
 inesoderm may also take part in the formation of the sensory 
 organs, giving rise to protective coverings and canals as well as to 
 contractile and nutritive elements (muscles, blood- and lymph- 
 channels). 
 
 In the sensory organs of the integument of Fishes as well as in 
 all the higher sensory organs the medium surrounding the end-organ 
 is always moist. In both cases, we meet with rod-, club-, or pear- 
 shaped sensory cells, but in the former the nerves coming from them 
 do not pass through nerve-cells, as they do in the organs of 
 higher sense. This indicates a lower stage of development, there 
 being no differentiation into sensory cell and nerve cell. 
 
 In those animals which in the course of development give up an 
 aquatic life and come on land (Amphibia) the external layers of the 
 epidermis dry up, and the integumentary sense-organs pass further 
 inwards from the surface, undergoing at the same time changes of 
 form. Thus from Reptiles onwards the rod-shaped end-cell no 
 longer occurs, and two kinds of nerve-endings are seen in the skin 
 terminal cells, and fine intercellular nerve-networks known as 
 free nerve-endings. 
 
 SENSE-OKGANS OF THE INTEGUMENT. 
 
 a. Nerve- eminences. 
 
 In Amphioxus certain rod-shaped or pear-shaped cells can 
 be recognised in the epidermis, especially in the anterior part 
 of the animal ; each of these is provided distally with a hair-like 
 process and proximally is in contact with a nerve. The cells 
 are distributed irregularly, but in the neighbourhood of the mouth 
 and cirri they tend to form groups. 
 
 It is doubtful whether these structures in Amphioxus are 
 directly comparable to. the integumentary sense-organs of Fishes 
 and Amphibians, but it is important to note that each of the latter 
 always arises in the first instance from a single cell which forms a 
 group by division. These organs always consist of central cells, 
 arranged in the form of a rounded and depressed pyramid, and 
 of a peripheral mass grouped around the former like a mantle. 
 The central cells are surrounded by a network of nerve-fibres ; 
 each of them bears at its free end a stiff cuticular hair, and they 
 are to be looked upon as the sensory cells proper. The others 
 function only as a supporting and slime-secreting mass (Figs 150 
 and 151). 
 
 In Dipnoi, aquatic Amphibia and all amphibian larva? these 
 organs retain throughout life their peripheral free position, on 
 
SENSE-ORGANS OF THE INTEGUMENT 191 
 
 a level with, the epidermis, 1 but in Fishes they may in post-em- 
 bryonic time become enclosed in depressions or complete canals, - 
 which are formed either by the epider- 
 mis only, or, as is more usually the case, 
 by the scales and bones of the head, and 
 which open externally. The organs are 
 thus protected. 
 
 These sensory organs are situated 
 characteristically along certain tracts, 
 the position of which is very constant : 
 in the head, supra-orbital, infra-orbital, 
 
 and liyomandibular tracts can be recog- 
 
 , J , , 7 . f FIG. loO. TRANSVERSE SEC- 
 
 nised, and a lateral line, (or several TIOX OF A F REELY p RO - 
 
 Proteus and all Amphibian larvae) ex- JECTING SEGMENTAL SENSE- 
 
 tends along the sides of the body to the ORGAN. 
 
 caudal fin (Figs. 152 and 153). They The cuticular tube and the 
 
 are thus often spoken of as segmental ^Zf SnTet c"! 
 
 sensory organs or organs of the lateral central (sensory) cells ;MZ, 
 
 line? The portions -lying in the region MZ 1 , peripheral cells. 
 
 of the head are innervated by the lateral 
 
 line branches of the facial, glossopharyngeal, and vagus (see note 
 
 on p. 185). 
 
 Freely projecting nerve-eminences are not present in Rays and 
 Ganoids, and are only of minor importance in Sharks. In all 
 these Fishes the integumentary sense-organs are more or less deeply 
 situated, being enclosed in complete or incomplete canals arising 
 as proliferations of the epidermis extending into the dermis, and 
 becoming greatly branched. 
 
 The so-called Savi's vesicles of Torpedo, the ''nerve sacs" of 
 Ganoids, and the ampullae of Elasmobranchs, correspond to modified 
 nerve-eminences. They are all limited in their distribution to the 
 head and anterior portion of the trunk, being most numerous on the 
 snout : they arise from thickenings of the epidermis which later 
 become invaginated and in which a sensory epithelium is differ- 
 entiated. In Ganoids these organs retain a simple sac-like form, 
 and in Torpedo they become completely separated off from the 
 epidermis, while in other Elasmobranchs they are tabular, each 
 tube giving rise to one or more swellings or ampullae, separated 
 
 1 At the time when an Amphibian undergoes metamorphosis and gives up its 
 aquatic habits, these sensory organs sink downwards into the deeper layer of the 
 skin, and, as the epidermis grows together over them, they become shut off from 
 the exterior and reduced, and may finally disappear. (Anura and certain Caduci- 
 branchiata. ) In other Urodeles they may, in some cases, be retained throughout 
 life, and are said to come to the surface when the animal returns to tne water 
 during the breeding season ; but, more usually, new organs then become 
 developed. 
 
 2 This is also the case on the head in Dipnoans. 
 
 3 In the Dipnoi they are not limited to the lateral line, and in Marsipobranchii 
 they have no regular arrangement and are not numerous, although a lateral 
 branch of the vagus is present. 
 
192 
 
 COMPARATIVE ANATOMY 
 
 / 
 / R 
 
 FIG. 151. NERVE ELEVATION OF A URODELE. (Semidiagrammatic.) 
 
 a, a, cells of the epidermis, through which the neiiro-epithelium, 6, 6, can be 
 seen ; c, the terminal hairs of the latter (the peripheral cells are not repre- 
 sented) ; R, hyaline tube, formed as a secretion ; 2v, the nerve-fibres passing 
 to and surrounding the sensory cells. 
 
 FIG. 152. SENSORY CANALS OF Chimcera monstrosa. (After F. J. Cole.) The 
 innervation is indicated by the different kinds of shading. 
 
 (1.) Supra-orbital canal (innervated by superficial ophthalmic of facial cross- 
 hatched the black segment is the portion innervated by the profundus) = 
 cranial (C) + rostral (R) + sub-rostral (SR). 
 
 (2.) Infra-orbital canal (buccal + otic of facial dotted) = orbital (Or) + sub- 
 orbital (SO) + portion of angular (A ) + nasal (N. ) 
 
 (3.) Hyomandibular or operculo-mandibular canal (external mandibular of facial 
 black )= remainder of angular (A) + oral (0) + jugular (J.) 
 
 (4.) Lateral canal (lateral line branch of vagus oblique shading) lateral (L) 4- 
 occipital (Oc) + aural (An) + post-aural (PAu.) 
 
SENSE-ORGANS OF THE INTEGUMENT 193 
 
 off from the rest of the tube by radial folds of connective tissue, 
 and containing the nerve-endings. The tubes are rilled with a 
 gelatinous substance. 
 
 The function of the nerve eminences is doubtful, but it appears 
 that they are concerned with the perception of mechanical stimuli 
 
 FIG. 153. DISTRIBUTION OF THE LATERAL SENSE-ORGANS IN A SALAMANDER 
 
 LARVA. 
 
 from the surrounding water, and thus are important as regards the 
 appreciation of the direction of these stimuli. 
 
 The horny wart-like structures arising periodically during the breeding 
 season in Cypriiioids and known as "pearl organs," are due to a modification 
 of the reduced nerve-eminences. Similar structures occur in Anura. 
 
 1). End-buds. 
 
 The nerve eminences pass through a stage in development in 
 which they clearly resemble end-buds, and the latter may be 
 looked upon as the phyletically older organs, which do not become 
 so highly differentiated as the former. No sharp line of demarca- 
 tion can, however, be drawn between the two, as all kinds of inter- 
 mediate forms are met with : they are here described separately 
 merely for the sake of clearness. 
 
 In contrast to the nerve-eminences, which tend to sink below 
 the surface, the end-buds usually form a dome-like elevation pro- 
 jecting above the general level of the epidermis. A central 
 sensory epithelium, provided with sensory hairs, and peripheral 
 supporting cells can be recognised, but the former are as long as 
 the latter. 
 
 In Lampreys and most Elasmobranchs they remain at a primi- 
 tive stage of development, but become of great importance in 
 Ganoids and Teleosts, in which they are scattered irregularly over 
 the whole body and are particularly numerous in the fins, lip- 
 folds, barbules, and mouth. From the Dipnoi onwards they become 
 limited to the oral and nasal cavities. In Dipnoi and Amphibia 
 they occur on the papillaB of the oral and pharyngeal mucous 
 membrane and tongue. In Reptiles their distribution is somewhat 
 more limited, and this is still further the case in Mammals, in 
 which, however, they are still found on the soft palate, on the walls 
 of the pharynx, and even extend into the larynx ; but here they 
 are most numerous on the tongue, where they occur, situated more 
 deeply, on the circumvallate and fungiform papillae as well as on 
 the papilla foliata, and function as organs of taste. 
 
 o 
 
194 
 
 COMPARATIVE ANATOMY 
 
 FIG. 154A. A TACTILE 
 
 SPOT FROM THE SKIN 
 
 OF THE FROG. Semi- 
 diagrammatic. (Modi- 
 fied from Merkel. ) 
 
 N, nerve, which loses its 
 medullary sheath at JV n ; 
 a, a, neuro-epithelium ; 
 ft, epidermis. 
 
 FIG. 154c. A TACTILE CORPUSCLE [(END-BULB) 
 FROM THE MARGIN OF THE.; CONJUNCTIVA OF 
 MAN. (After Dogiel. ) 
 
 n, medullated nerve fibre, the axis-fibre of 
 which passes into a closely coiled terminal 
 skein ; b, nucleated fibrous investment. 
 
 FIG. 154B. DERMAL PAPILLA 
 FROM THE HUMAN FINGER 
 
 ENCLOSING A TACTILE CoR- 
 
 PUSCLE. (After La wdowski.) 
 
 a, fibrous and cellular invest- 
 ment ; />, tactile corpuscle, 
 with its cells ; M, nerve-fibre ; 
 n', the further course of the 
 nerve-fibre, showing its 
 curves and bends ; n", termi- 
 nal twigs of the nerve-fibres 
 with club-shaped endings. 
 
 FIG. 154D. TRANSVERSE SECTION 
 THROUGH A TACTILE CORPUSCLE 
 FROM THE BEAK OF A DUCK. 
 
 (After Carriere.) 
 
 n, nerve, entering the capsule K, 
 its sheath (S) becoming con- 
 tinuous with the latter. The 
 nerve passes between the two 
 covering-cells, DZ, DZ, widen- 
 ing out to form a tactile plate 
 at n\- 
 
CLUB-SHAPED CORPUSCLES 195 
 
 c. Tactile-cells and corpuscles. 
 (Terminal ganglion cells.) 
 
 In these structures there is no longer any direct connection 
 with the surface of the epidermis, and supporting cells are want- 
 ing. 
 
 " Tactile spots," consisting of groups of touch cells, are met with 
 for the first time in tailless Amphibians, in which they are situated 
 mainly on small elevations, and are distributed over the skin of the 
 whole body (Fig. 154 A). In Reptiles they are found chiefly in 
 the region of the head, on the lips and sides of the face, and on 
 the snout, but in some cases (as in Blindworms and Geckos), they 
 extend over the whole body close to the scales. In Snakes and 
 Birds the tactile cells are confined to the mouth-cavity (tongue) 
 and to the beak (cere), and lie much more closely together, forming- 
 definite masses, or tactile corpuscles (Fig. 154D). Each of these is 
 surrounded by a nucleated connective-tissue investment, from which 
 septa extend into the interior, partially separating the individual 
 tactile cells from one another. In Mammals the tactile cells are 
 either isolated as, for instance, on the hairless portions of the body, 
 or they give rise to oval corpuscles, each consisting of a many- 
 layered and nucleated investment, into which a nerve passes, be- 
 comes twisted up, and comes into relation with one or more ter- 
 minal cells (Fig. 154 B, c). These are most numerous and highly 
 developed on the volar and plantar surfaces of the hand and foot 
 respectively, and on the conjunctiva and snout. 
 
 d. Club-shaped corpuscles. 
 (Pacinian corpuscles.) 
 
 From the Reptilia (Lizards, Snakes) onwards, club-shaped 
 corpuscles are present in addition to the above-described tactile- 
 organs. In these Reptiles they occur chiefly in the region of the 
 lips and teeth ; they have an elongated, oval form, and their structure 
 is simple. 
 
 In the interior of each corpuscle is seen the continuation of 
 the axis-fibre of the nerve which becomes swollen distally, and 
 externally to this is a double column of cells which enclose the 
 club-shaped axis (Fig. 155). It is probable that a fine branch 
 is given off from the axis-fibre to each cell. The column of cells 
 is enclosed externally by an investment consisting of numerous 
 nucleated lamellae in which longitudinal and circular layers can 
 be distinguished. 
 
 Organs of this kind are universally present, deeply situated, 
 
 o 2 
 
196 
 
 COMPARATIVE ANATOMY 
 
 in the skin of Birds and Mammals, and in the former they are 
 particularly abundant on the beak and at the bases of the con- 
 tour-feathers of the wings and tail, 
 and are also found on the tongue. 
 They occur, moreover, in various 
 other regions, both in Birds and 
 Mammals (e.g. the various organs 
 of the abdominal cavity, the con- 
 junctiva, the fasciae, tendons, liga- 
 ments, vas deferens, periosteum, peri- 
 cardium, pleura, corpus cavernosum 
 a.nd spongiosum, the wing-membrane 
 of Bats, &c.). 
 
 The tactile cells and tactile and 
 club-shaped corpuscles are all con- 
 cerned with the sense of touch. It 
 is impossible to say definitely what 
 nerve-endings have to do with the 
 perception of temperature ; it is not 
 improbable that the touch cells, as 
 well as the nerve-fibres often pro- 
 vided with varicose swellings which 
 end freely in the epidermis, are con- 
 cerned in this process. Such free 
 nerve-endings occur in the skin of 
 all Vertebrates, and consist of an 
 intercellular network, no direct con- 
 nection between nerve and epithelial 
 cell having been observed. 
 
 FlG. 
 
 155. A PACINIAN 
 PUSCLE. 
 
 COB- 
 
 A , axis fibre ; A 1 , tufted or knob- 
 like end of the same ; NS, nuc- 
 leated sheath of nerve, which 
 passes into the external longi- 
 tudinal series of lamellse, L ; Q, 
 internal, circular layer of the 
 external part of the club ; JK, 
 internal part of the club formed 
 of the cell-pillars. 
 
 OLFACTOEY ORGAN. ' 
 
 The olfactory lobe as already mentioned (p. 153) represents a pro- 
 longation of the secondary fore-brain, the ventricle of which is tem- 
 porarily or permanently continued into it. In some cases it becomes 
 differentiated into olfactory bulb, tract, and tubercle (pp. 159-175). 
 
 The olfactory nerves proper are connected with the bulb, and are 
 usually arranged in a single bundle on either side, with more or less 
 distinct indications of a subdivision into two bundles : they ap- 
 parently arise in continuity with the epithelium of the nasal 
 involution (comp, pp. 177, 187) and then grow centripetally, 
 uniting with the olfactory lobe or bulb secondarily. 
 
 In all Mammals but Ornithorhynchus, as well as in Menopoma, 
 Apteryx, and the extinct Dinornis, the olfactory nerves pass into 
 this nasal cavity separately, through a cribriform plate of the 
 ethmoid (p. 99). 
 
OLFACTORY ORGAN 
 
 197 
 
 The primary origin of the olfactory organs is by no means understood : 
 possibly it may have arisen by a modification of primitive integumentary 
 sense-organs. It is doubtful whether the organ can be said to have a true 
 olfactory function in Fishes and perennibranchiate Amphibians. 
 
 In its simplest form, the olfactory organ consists of a ventral, 
 paired, pit-like depression of the integument of the snout opening 
 on to the surface by an external nostril. It is lined by an epithelium 
 which is connected with the brain by the olfactory nerves. The 
 olfactory mucous membrane contains sensory cells, or olfactory cells 
 proper usually provided with sensory 
 
 hairs, separated by isolating or supporting R^li, 
 
 cells, both kinds having a smilar origin 
 (Fig. 156). A W 
 
 These olfactory cells are said to constitute the 
 only true neuro-epithelium in Vertebrates, as the 
 nerve arises in connection with the cell itself, 
 with which it remains continuous, as in many 
 Invertebrates (primary sensory cell, Retzius). The 
 cell is therefore not merely surrounded by a 
 nerve-network as in other secondary nerve-cells, 
 and the olfactory organ thus probably represents 
 a very ancient structure phylogenetically. It is 
 possible, however, that the central cells of the in- 
 tegumentary sense-organs of Anamnia (e.g. end- 
 buds) may be directly continuous with their 
 nerves, although surrounded by a nerve -net work. 
 
 From the Amphibia onwards glandular 
 elements are present, the secretion of which 
 serves to keep the nasal cavity moist. 
 
 The olfactory organs of all the true 
 Fishes exhibit the above-described simple 
 sac-like form, but from the Dipnoi onwards 
 they come to communicate with the cavity 
 of the mouth as well as with the exterior. 
 In consequence of this, anterior or external, 
 
 and posterior or internal nostrils (choance) can be distinguished, 
 and as a free passage is thus formed through which air can pass, 
 the olfactory organ takes on an important relation to the respira- 
 tory apparatus. 
 
 In Amphioxus, the ciliated pit situated above the anterior end of the 
 central nervous system probably represents an unpaired olfactory organ. 
 Traces of a structure possibly homologous with this are said to occur in the 
 embryos of the Lamprey and Sturgeon. 
 
 Cyclostomes. In these forms (Fig. 54) the olfactory organ 
 consists of a sac, containing numerous radial folds of the mucous 
 membrane, and unpaired externally. It lies close in front of the 
 cranial cavity, and opens on the dorsal surface of the anterior 
 part of the head by a longer or shorter chimney-like tube. In. 
 
 FIG. 156. EPITHELIUM 
 OF THE OLFACTORY 
 Mucous MEMBRANE. 
 A, of Petromyzon plan- 
 er i ; B, of Salamandra 
 atra. 
 
 R, olfactory 
 interstitial 
 cells. 
 
 cells ; E, 
 epithelial 
 
198 
 
 COMPARATIVE ANATOMY 
 
 Myxine this tube is long, and is supported by rings of cartilage. In 
 the larval lamprey the organ is at first ventral and unpaired (Fig. 
 125), but subsequently becomes sunk in a common pit with the 
 pituitary imagination and takes on a dorsal position : it is almost 
 completely divided into two lateral halves internally by the forma- 
 tion of a fold of the mucous membrane. The pituitary sac thus 
 extends backwards from the ventral side of the organ above the 
 mucous membrane of the mouth : in Petromyzon it ends blindly, 
 but in Myxine it opens into the oral cavity, perforating the skull 
 floor from above instead of from below as in other Vertebrates. 
 
 Fishes. : The position of the olfactory organ in Elasmobranchs 
 (Fig. 157, A) differs from that seen in Cyclostomes in lying on the 
 
 FIG. 157. A, VENTRAL VIEW OF THE HEAD OF A DOGFISH (Scyllium canicula). 
 N, external nostril ; M, mouth ; HSO, integumentary sense-organs. 
 
 B, LATERAL VIEW OF THE HEAD OF A PIKE (Esox lucius). a and b, the anterior 
 and posterior openings of the external nostrils ; f, fold of skin separating a 
 and b ; Ag, eye. 
 
 under instead of the upper surface of the snout, and thus retains 
 the more primitive position. From these Fishes onwards the 
 organ is always paired, each sac being more or less completely 
 enclosed by a cartilaginous or bony investment forming an outwork 
 of the skull. 
 
 From the Ganoids onwards it always has a similar position with 
 regard to the skull, being situated between the eye and the end of 
 the snout, either laterally or more or less dorsally : originally, 
 however, it is ventral. In the course of development each external 
 nostril of Ganoids and Teleosts becomes completely divided into 
 
OLFACTORY ORGAN 
 
 199 
 
 "two portions, an anterior and a posterior (Figs. 157, B, and 158), by 
 a fold of skin. The nostril often lies at the summit of a longer or 
 .shorter tube, lined with ciliated cells, and the distance between 
 the anterior and the posterior aperture varies greatly, according to 
 the width of the fold of skin which separates them. 
 
 The mucous membrane of the nasal organ of Fishes is always 
 raised up into a more or less complicated system of folds, which 
 .may have a transverse, radial, rosette-like, or longitudinal arrange- 
 
 ESO 
 FIG. 158. LATERAL VIEW OF THE HEAD OF Murwna helena. 
 
 VR and HR, anterior and posterior tubes of the external nostrils ; A, eye ; 
 HSO, integumentary sense-organs. 
 
 ment, and which are supplied by the branches of the olfactory 
 nerve. 
 
 The olfactory organ of Polypterus is more highly developed and compli- 
 cated than that of any other Fish. In certain representatives of the Plecto- 
 gnathi and Gymnodontes amongst the Teleostei, on the other hand, the organ 
 shows various stages of degeneration. 
 
 Dipnoi. A nasal skeleton well differentiated from the skull 
 proper is met with for the first time in Dipnoans. In Protopterus 
 it consists of a cartilaginous trellis-work enclosing each olfactory 
 sac and united with its fellow in the median line by a solid septum : 
 the floor is formed mainly by the pterygopalatine and by con- 
 nective tissue. The mucous membrane is raised into numerous 
 transverse folds connected with a longitudinal fold, and the olfac- 
 tory organ in general most nearly resembles that of Elasmobranchs, 
 
200 
 
 COMPARATIVE ANATOMY 
 
 except that, as already mentioned, posterior (internal) as well as 
 anterior (external) nostrils are present, The latter open beneath 
 the upper lip, and so cannot be seen when the mouth is closed ; 
 the former open into the oral cavity rather further back. 
 
 The peculiar position of the anterior nares has a physiological significance, 
 at any rate in Protopterus, in connection with its habits (see p. 17) ; during 
 its summer sleep the animal breathes through a tube, passing between the 
 lips, formed from the .capsule or cocoon which encloses it. The necessary 
 moisture for the olfactory mucous membrane during this time is provided 
 by the numerous goblet cells which line the walls of both nostrils. 
 
 Amphibia. The olfactory organ of the Pereiinibranchiata re- 
 sembles in many respects that of the Dipnoi : it is always enclosed 
 
 within a complete or perforated 
 cartilaginous capsule situated 
 laterally to the snout close be- 
 neath the skin, and is not pro- 
 tected by the bones of the skull 
 (Fig. 159). Its floor is largely 
 fibrous, and the mucous mem- 
 brane is raised into radial folds 
 like those of Cyclostomes and 
 Polypterus. In all the other 
 Amphibia it becomes included 
 within the cranial skeleton, and 
 lies directly in the longitudinal 
 axis of the skull in front of the 
 cranial cavity. 
 
 The structure of the olfac- 
 tory organ now becomes modified 
 in correspondence with the change 
 in the mode of respiration ; the 
 nasal chamber becomes differ- 
 entiated into an olfactory and a 
 respiratory portion, and an ex- 
 tension of the olfactory surface takes place by the formation 
 of one or more prominences on the floor and side-walls of 
 the nasal cavity. These prominences, which may be compared 
 to the turbinals of higher forms, are present in certain Mycto- 
 dera (Fig. 160), and attain a very considerable development in 
 Anura and Gymnophiona, especially in the latter, in which the 
 nasal chamber is converted into a complicated system of spaces 
 and cavities. A main chamber and a more laterally situated 
 accessory cavity can in all cases be distinguished, even in the 
 Derotremata and Myctodera ; the accessory cavity lies in the 
 maxillary bone (Fig. 160 and 165 A E). 
 
 In certain Gymnophiona the accessory chamber becomes entirely shut off 
 from the main cavity and receives a special branch of the olfactory nerve, 
 so that in these cases two separate nasal cavities can be distinguished. 
 
 ORGAN OF 
 (From the 
 
 FIG. 1 59. OLFACTORY 
 Nectunut maculatus. 
 dorsal side. ) 
 
 A r , olfactory sac ; 01, olfactory nerve ; 
 Pmz, premaxilla ; F, frontal ; P, 
 process of the parietal ; PP, palato- 
 pterygoid ; AF, antorbital process. 
 
OLFACTORY ORGAN 201 
 
 Glands, situated under the olfactory mucous membrane, are now 
 also met with ; these are either diffused, or united to form definite 
 masses. They either open directly into the nasal cavity, their 
 secretion serving for the necessary moistening of the mucous mem- 
 brane (effected in Fishes by the external medium), or they pour 
 their secretion into the pharynx or posterior nostrils. The latter 
 always lie tolerably far forwards on the palate, and are for the most 
 part enclosed by the vomer, as well as the palatine. 
 
 Finally, the naso-lachrymal duct must be mentioned : it passes 
 out from the anterior angle of the orbit, through the lateral wall 
 
 FIG. 160. TRANSVERSE SECTION THROUGH THE OLFACTORY CAVITIES OF 
 Plethedon glutiiiosus (Myctodera). 
 
 S, S, olfactory mucous membrane ; JV, main nasal cavity ; K, maxillary cavity ; 
 C, cartilaginous, and AS' 1 , fibrous portion of the turbinal, which causes the 
 olfactory epithelium (E) to project far into the nasal cavity ; ID, inter- 
 maxillary gland, shut off from the cavity of the mouth by the oral mucous 
 membrane (MS) ; F, frontal ; Pf, pref rental ; M, maxilla ; Vop, vomero- 
 palatine ; Sp, nasal septum. 
 
 of the nose, and opens into the nasal cavity on the side of the 
 upper jaw. It conducts the lachrymal secretion from the conjunc- 
 tival sac of the eye into the nasal cavity, and arises in all Verte- 
 brates, from the Myctodera onwards, as an epithelial cord which is 
 separated off from the epidermis, and, growing down into the 
 dermis, becomes hollow secondarily. 
 
 Reptilia. Owing to the growth of the brain and facial region 
 and to the formation of a secondary palate (p. 92), the olfactory 
 organs, from Reptiles onwards, gradually come to be situated more 
 ventrally, beneath the cranium. 
 
 The Lacertilia, Ophidia, and many Chelonia possess the sim- 
 plest olfactory organs amongst Reptiles. The nasal cavity of 
 Lizards is divided into two portions, a smaller outer (anterior), and 
 a larger inner (posterior) or olfactory chamber proper. The latter 
 only is provided with sensory cells, the former being lined by 
 ordinary stratified epithelium continuous with the epidermis and 
 containing no glands. A large turbinal, slightly rolled on itself, 
 arises from the outer wall of the inner nasal chamber, and extends 
 far into its lumen ; this is also well developed in Ophidia, in 
 which a distinct outer nasal chamber is wanting ; it may be 
 derived from that of the Amphibia. 
 
"202 
 
 COMPARATIVE ANATOMY 
 
 A large gland which opens at the boundary between the inner 
 and outer nasal cavities lies within the turbinal. Below the latter 
 is the aperture of the lachrymal duct : this duct in some Reptiles 
 opens on the roof of the pharynx (Ascalabota), and in others into 
 the internal nostrils (Ophidia). 
 
 The structure of the nose in Chelonians is very complicated and varied. 
 In marine Chelonians it is divided into two passages, one above the other, 
 
 and connected by means of a perforation of the 
 septum. The comparative paucity of glands in 
 
 AN +^-- -~T~~ *ke lf actor y or g an of Lizards and Snakes forms 
 
 '-"'"*' a marked contrast to the condition seen in 
 
 Chelonians, the nasal organ of which is charac- 
 terised by a great abundance of them. 
 
 The extension downwards and back- 
 wards of the olfactory organ is most marked 
 in Crocodiles, in correspondence with the 
 
 OLFACTORY ORGAN OF A growth forwards of the facial region and 
 LIZARD. (Longitudinal the formation of the palate ; its posterior 
 part thus comes to lie below the brain 
 and base of the skull. Each nasal chamber 
 
 FIG. 161. DIAGRAM OF THE 
 
 vertical section.) 
 
 AN, IN, outer and inner 
 nasal chambers ; t, tube- 
 like connection between 
 
 nasal chambers ; t, tube- j s divided posteriorly into two superim- 
 
 them ; Ch, internal nos- P osed cavities, the upper of which repre- 
 trils; P, papilla of Jacob- sents the proper olfactory chamber, and is 
 .son's organ ; Ca, aperture i ine( j by sensory epithelium, while the 
 
 of communication of the , F > - 
 
 latter with the mouth; lower functions as a respiratory portion 
 
 MS, oral mucous mem- only. Certain accessory air-chambers are 
 
 ^ rane - connected with the nasal cavity. A large 
 
 gland is present between the olfactory 
 
 chamber and its investing bones, and opens into the nasal cavity. 
 As in other Reptiles, there is only a single true turbinal, but ex- 
 ternally to it lies a second prominence, which may be spoken of as 
 a pscudo -turbinal. 
 
 Birds. In all Birds, as in Lizards, there is an outer chamber, 
 lined by stratified epithelium, and a proper olfactory chamber, 
 situated above the former. Birds also possess only a single true 
 turbinal if by this term is understood a free independent projection 
 into the nasal cavity supported by skeletal parts. Two other pro- 
 minences (pseudo-turbinals) are, however, present ; one of which lies, 
 like the true turbinal, in the proper olfactory chamber, while the 
 other, like the pseudo-turbinal of the Crocodile, is situated in the 
 outer portion : these are simply incurved portions of the whole 
 nasal wall (Fig. 162). 
 
 The form of the true turbinal, which is usually supported by 
 -cartilage more rarely by bone, varies greatly. It is either repre- 
 sented by a moderate-sized prominence, or else it becomes more 
 or less rolled on itself. The lachrymal duct opens below and an- 
 
OLFACTORY ORGAN 
 
 203 
 
 teriorly to it. This turbinal is comparable to that of Urodeles and 
 Reptiles. 
 
 The so-called external nasal gland of Birds is situated on the 
 frontal or nasal bones, along the upper margin of the orbit. It 
 is supplied by the first and second branches 
 of the trigeminal, and corresponds to the 
 lateral nasal gland of Lizards. 
 
 Mammals. Corresponding to the much 
 more marked development of the facial por- 
 tion of the skull, the nasal cavity of Mammals 
 is proportionately much larger than in the 
 forms yet described, and consequently there 
 is much more room for the extension of the 
 turbinals : these give rise to a spongy laby- 
 rinth, the cell-like compartments of which 
 .are lined by mucous membrane ; and thus 
 variously shaped projections, supported partly 
 by cartilage and partly by bone, are seen ex- 
 tending into the nasal cavity (Fig. 163, A c). 
 
 The normal number of these true olfactory 
 ridges or scrolls varies considerably. 1 They 
 may be arranged in one row (Orni thorny n- 
 chus, Cetacea, Pinnipedia, Primates), or in 
 several rows (other Mammals), in which latter 
 case the olfactory lobes are largely developed. 
 
 According to the degree of development of the olfactory appa- 
 ratus, taking specially into account its cerebral portion (olfactory 
 lobes), we may distinguish between Mammals which are macros- 
 matic (the majority of the mammalian orders), microsmatic (Seals, 
 Whalebone Whales, Monkeys, Man), and anosmatic (most Toothed 
 Whales). 
 
 The above-mentioned olfactory scrolls belong to the true olfac- 
 tory region, and are generally described as " ethmoid turbinals" 
 as in all but the first of the series their skeletal supports usually 
 become united with the ethmoid bone, the first coming into rela- 
 tion with the nasal, and being therefore usually spoken of as the 
 " nasal turbinal" It must, however, be borne in mind that these 
 do not correspond to the turbinals of lower Vertebrates. The latter 
 -are represented by the so-called " maxillary turbinal" situated in 
 the anterior (lower) portion of each nasal chamber, which com- 
 municates with the pharynx by the internal nostrils, its skeletal 
 portion becoming united with the maxillary bone (Fig. 163, c). 
 This maxillary turbinal no longer possesses an olfactory epithelium 
 
 1 In most of the Mammalian orders, five olfactory scrolls are typically present ; 
 in Echidna there are six or more ; in Ungulates there may be as many as 
 eight ; amongst Edentates, Orycteropus possesses eleven ; while in adult 
 Primates there are only from one to three, a greater number being present in the 
 mbryo (Fig. 163). 
 
 FIG. 162. TRANSVERSE 
 SECTION THROUGH THE 
 RIGHT NASAL CAVITY 
 OF A SHRIKE (Laniu* 
 minor. ) 
 
 OM, MM, superior 
 (pseudo) and middle 
 (true) turbinal ; a, 
 upper, and b, lower 
 nasal passage ; LR, 
 air - chamber, which 
 extends into a hollow 
 of the superior tur- 
 binal. 
 
204 
 
 COMPARATIVE ANATOMY 
 
 after the embryonic period, 1 and has plainly undergone a change of 
 function in connection with the perception of the warmth and 
 
 moisture of the inspired air. 
 When well developed, it forms 
 
 and 
 less 
 
 single 
 
 OS, 
 
 FIG. 163, A. ^LATERAL VIEW OF THE 
 NASAL CHAMBER OF A HUMAN EM- 
 BRYO. 
 
 /, //, ///, the three olfactory ridges ; f, 
 supernumerary ridge which occurs in 
 the embryo ; it, tip of the nose ; pi, 
 hard palate ; cr, base of skull 
 Eustachian aperture. 
 
 nasal apparatus, but may 
 lose their primary func- 
 tion, often persisting merely 
 as air-sinuses. 
 
 The nasal glands may 
 be divided into two sets, 
 numerous small, diffuse 
 Boivmans glands, and a 
 large gland of Stenson. 
 The latter appears early in 
 the embryo, and often be- 
 comes greatly reduced later 
 on in development ; it is 
 situated in the lateral or 
 basal walls of the nasal 
 cavity, and may extend 
 into the maxillary sinus 
 when the latter is well 
 developed. 
 
 The appearance of an 
 external nose is very charac- 
 teristic of the olfactory or- 
 gan of Mammals : this must 
 be regarded as a derivative 
 of the outer chamber of the 
 
 a single or double coil, 
 may even be more or 
 branched (Fig. 164). Branches 
 of the trigemiual extend over 
 it, and supply its mucous mem- 
 brane. An olfactory and a 
 respiratory region can there- 
 fore also be distinguished in the 
 nasal chamber of Mammals. 
 
 The nasal chamber usually 
 communicates with neighbour- 
 ing cavities, such as the maxil- 
 lary, frontal, and sphenoidal 
 sinuses (Fig. 163, B, c) : the 
 two last-mentioned cavities 
 arise in connection with the 
 
 FKJ. 163, B. SAGITTAL SECTION THROUGH THE 
 NASAL AND BUCCAL CAVITIES OF THE 
 HUMAN HEAD. 
 
 /, //, ///, the three olfactory ridges ; sn', 
 Jrontal sinus ; an", sphenoidal sinus ; o-s, 
 aperture of Eustachian tube ; be, entrance 
 to the mouth ; Ig, tongue ; ?:. i, atlas verte- 
 bra ; v.ii, axis vertebra. 
 
 nose of Reptiles and Birds. 
 
 1 This is also true of the anterior (lower) ethmoid turbinal in microsmatic 
 Mammals. 
 
OLFACTORY ORGAN 
 
 {05 
 
 It is supported by an outward 
 cartilaginous septum nasi 
 which arises from the eth- 
 moid, as well as by other 
 secondarily independent car- 
 tilages (ali-nasals) which 
 were primarily continu- 
 ous with the general carti- 
 laginous wall, but become 
 differentiated from the 
 latter in various ways in 
 accordance with the varied 
 functional adaptations which 
 the outer nose undergoes. 
 Thus it may be provided 
 with a special valvular ap- 
 paratus for closing the nos- 
 trils (aquatic Mammals) ; or 
 may grow out to form a 
 longer or shorter trunk 
 provided with a complicated 
 musculature (Mole, Shrew, 
 Pig, Tapir, Elephant), and, 
 by means of its abundant 
 nerve-supply, serve as a 
 delicate organ of touch and 
 even as a prehensile appar- 
 atus. 
 
 n 
 
 extension of the nasal bones and by the 
 
 FIG. 163, c. TRANSVERSE VERTICAL SECTION 
 
 THROUGH THE NASAL CAVITY OF MAN. 
 
 /, //, ///, inferior (maxillary), middle, and 
 superior turbinal ; a, b, c, inferior, middle, 
 and superior nasal passage ; S, septum 
 nasi ; J, J, position of rudimentary Jacob- 
 son's organs, which are situated nearer the 
 floor of the cavity than is indicated in the 
 figure ; *, point at which the naso-lachrymal 
 duct opens ; t, entrance into the maxillary 
 sinus (C.m) ; SL, ethmoidal labyrinth ; 
 ffG, hard palate ; C.cr, cranial cavity ; 
 J/, maxilla ; Or, wall of orbit. 
 
 D 
 
 E 
 
 FIG. 164. VARIOUS FORMS OF THE MAXILLO-TURBINAL OF MAMMALS. 
 
 A , double coil ; B, transition from latter to single coil, E, F ; C, transition from 
 double coil to the dendritic form D. (After Zuckerkandl. ) 
 
 JACOBSON'S ORGAN. 
 
 By the term " Jacobson's organ " is understood a paired accessory 
 nasal cavity which in an early embryonic stage becomes differen- 
 tiated from the nasal chamber, and which is supplied by the 
 olfactory and trigeminal nerves; it communicates with the mouth 
 by a special aperture. 
 
f/.m. 
 
 
 
 
 c. 
 
 1. G i 
 
 
 g. m . 
 
 __ _ -^>~~i__ t ~ ~"^ ^_ ~~ _. ~^" 
 
 FIG. 165. TRANSVERSE SECTIONS OF THE NOSE IN VARIOUS VERTEBRATES. 
 A D, Illustrating the various ontogenetic and phylogenetic stages of the Jacob- 
 son's organ of Urodeles. In A its position is niedian, and in D lateral. 
 
 E, Gymnophiona, in which the organ becomes separated from the main nasal cavity. 
 
 F, Lacerta agilis. 
 
 G, Placental Mammal ; I, the same, in longitudinal vertical section. 
 H, Ornithorhynchus. (After Symington. ) 
 
 N, main nasal cavity; jc, Jacobson's organ; c.j, Jacobson s cartilage; y.m, 
 intermaxillary gland ; g.n, nasal gland ; n.o, olfactory nerve ; n.t, trigeminal 
 nerve ; d.n, naso-lachrymal duct; mx, maxilla ; o.d, dumb-bell shaped bone. 
 
EYE 207 
 
 A Jacobson's organ is first met with in Amphibians. In young 
 Triton larvae a small gutter-like medio-ventral outgrowth of each 
 nasal cavity arises, with which the ventral branch of the olfactory 
 nerve comes into relation. This outgrowth later undergoes a re- 
 lative change of position, and comes to be situated laterally towards 
 the upper jaw (Fig. 165 A D). At its blind end a gland is developed. 
 In Siren the primary median position is retained, and in the Axolotl 
 it does not extend so far laterally as in the adult Triton. The acces- 
 sory nasal chamber of Coecilians ] (p. 200) is developed in a similar 
 manner (E), and a large gland is in connection with it. There can 
 also be little doubt that this cavity is represented in Anura, 
 although its relative position is somewhat different to that seen in 
 Urodeles. 
 
 The Jacobson's organ of the Amniota is also developed in the 
 medio-ventral part of the nasal chamber, close to the septum nasi. 
 It loses its primary connection with the former, but retains its 
 median position, lying between the floor of the nasal cavity and 
 the roof of the mouth. It is lined by an olfactory epithelium and 
 communicates in front with the mouth through the corresponding 
 naso-palatine canal (p. 100). In Lacertilia and Ophidia a papilla 
 extends into its cavity from the floor (Figs. 161 and 165, F). 
 
 These organs are not present in Crocodiles, Chelonians, and 
 Birds, but rudiments have been observed in embryos of Crocodilus 
 biporcatus, and certain cartilages on the nasal floor in Birds 
 appear to correspond with the Jacobson's cartilages of other forms. 
 
 Amongst Mammals, Jacobson's organ is most marked in 
 Monotremes (Fig. 165, H), in which it is much more highly developed 
 than in Lizards. It contains a well-marked, turbinal-like ridge, 
 supported by cartilage continuous with that enveloping the organ 
 arid covered with ciliated epithelium, and numerous glands are 
 present in the mucous membrane. In other Mammals (G, l) it 
 becomes more or less reduced, though often well-marked, consisting 
 of two tubes lying at the base of the septum nasi, usually enclosed 
 by separate cartilages (Marsupials, Edentates, Insectivores, Rodents, 
 Carnivores, Ungulates). A branch of the olfactory nerve enters the 
 tube posteriorly, and anteriorly the cavity of the organ communi- 
 cates with the mouth through the incisive or naso-palatine canals. 
 Rudiments of the organ exist even in Man (Fig. 163, c). 
 
 The function of Jacobson's organ may consist in bringing the 
 food taken into the mouth under the direct control of the olfactory 
 nerve. 
 
 EYE. 
 
 As already mentioned (p. 154, Fig. 167, A and B), the optic 
 nerve is developed from the stalk of an outgrowth of the primary 
 
 1 A curious apparatus exists in Ccecilians in connection with the nasal cavity 
 and orbit. It consists of a fibrous capsule with muscles and a large gland, 
 opening near the snout. Its function is not certainly known. 
 
208 
 
 COMPARATIVE ANATOMY 
 
 fore-brain known as the primary optic vesicle. It, therefore, like the 
 olfactory lobe, represents a part of the brain. 
 
 In the adult brain, the optic nerve is seen to arise from the 
 thalamencephalon, and three more or less sharply-differentiated 
 portions of it may in most cases be distinguished; these are 
 spoken of, from the proximal to the distal end 
 respectively, as the optic tract, chiasma, and 
 nerve. 
 
 The chiasrna, that is, the crossing of the 
 two optic nerves, is always present, though 
 not always freely exposed, for.it may retain 
 a primitive position deeply embedded in the 
 base of the brain, (e.g., Cyclostomi, Dipnoi). 
 
 In most Teleosts the optic nerves simply 
 overlie one another (Fig. 166, A), but in some 
 of these Fishes (Clupea, Engraulis, Fig. 
 166, B), one nerve passes through a slit in the 
 other, and this condition of things is gradu- 
 ally carried still further in Reptiles, until 
 finally the fibres of the two nerves intercross 
 in a very complicated manner (Fig. 166, c, D), 
 giving rise to a sort of basket-work ; this is 
 finest and most delicate in Mammals, where 
 its structure can only be analysed by compar- 
 ing a series of sections. A more or less 
 complete crossing of the fibres of each optic 
 nerve may also take place more peripherally 
 before they spread out in the retina. 
 
 In contrast to the eyes of Invertebrates, 
 which arise by a differentiation of the cells 
 of the superficial epiblast, the sensitive ele- 
 mimber of Teleostei ; ments of the Vertebrate eye correspond to a 
 B,Herrin g ;C,Lacer- peripheral portion of the brain (Figs. 167, A 
 and B). 
 
 As the primary optic vesicle OTOWS out- 
 wards towards the outer skin of the embryo, 
 the portion which connects it with the brain 
 becomes constricted and by degrees loses its 
 cavity, giving rise to a solid cord, from which 
 the optic nerve is formed. 
 
 At the point where the vesicle touches 
 
 the epiblast, the latter becomes thickened and the outer 
 wall of the vesicle invaginated to form a double-walled cup. 
 the secondary optic vesicle (Fig. 167, B). The inner and 
 outer walls of the cup then become fused together, the 
 former giving rise to the sensory epithelium of the retina, and 
 the latter to the pigment epithelium. The fibres of the optic 
 nerve are first differentiated in its retinal portion, and grow 
 
 FIG. 166.- CHIASMA or 
 THE OPTIC NERVES. 
 (Semidiagrammatic.) 
 A, chiasma charac- 
 teristic of the greater 
 
 ta agilis ; D, an Ag- 
 ama ; E, a higher 
 Mammal. 
 
 Chi, chiasma of the 
 bundle of nerves ly- 
 ing centrally ; Ce, 
 Ce\ S, S 1 , lateral 
 fibres; Co, commis- 
 sure. 
 
EYE 
 
 centripetally towards the brain ; centrifugal fibres also arise 
 later. 
 
 In the course of further development, the epiblastic thickening 
 mentioned above, which is often at first hollow, becomes separated 
 from the epiblast, sinks more and more into the interior of the 
 optic vesicle, and is differentiated to form the crystalline lens 
 (Fig 167, B). The remaining space within the optic vesicle becomes 
 filled by mesoblastic tissue, which grows in from the ventral side 
 of the secondary optic vesicle through the so-called choroid fissure 
 
 -M* 
 
 FIG. 167, A. DIAGRAM SHOWING THE MODE OF FORMATION OF THE PRIMARY 
 OPTIC VESICLES (ABL) 
 
 VH, fore-brain ; V, V, ventricular cavity of the brain, which communicates freely 
 with the cavities of the primary optic vesicles at ft. 
 
 B. SEMIDIAGRAMMATIC FIGURE OF THE SECONDARY OPTIC VESICLE, AND OF 
 
 THE LENS BECOMING SEPARATED OFF FROM THE EPIBLAST. 
 
 IB, inner layer of the secondary optic vesicle, from which the retina arises ; 
 t, point at which the latter is continuous with the outer layer (AB), from 
 which the pigment epithelium is formed ; ff, remains of the cavity of the 
 primary optic vesicle ; L, lens, which arises as a cup-shaped involution of the 
 epiblast (E] ; *, point of involution of epiblast to form the lens ; MM, meso- 
 blastic tissue, which at M 1 , J/ 1 , grows in between the outer epiblast and the 
 lens as the latter becomes separated off, and which gives rise to the cornea as 
 well as to the iris ; C, vitreous chamber of the eye, between the lens and 
 retina, which later becomes filled by the vitreous humour. 
 
 and gives rise to the vitreous humour (Fig. 167, B), the bulk 
 of which, as compared with the lens, gradually increases. Blood- 
 vessels (vasa centralia nervi optici, arteria hyaloidea, tunica vasculosa 
 lentis) also extend into the vesicle in the same manner. 
 
 The secondary optic vesicle is thus plentifully supplied with 
 blood-vessels in its interior, and others arise at its periphery, where 
 a definite vascular and pigmented membrane, the choroid, is formed 
 from the surrounding mesoblast (Fig. 168). 
 
 Internally to the lens, the choroid gives rise to the ciliary 
 folds, while more externally it passes in front of the lens to form 
 
210 
 
 COMPARATIVE ANATOMY 
 
 the iris (Fig. 168), which retains in the centre a circular or slit-like 
 aperture, the pupil, through which the rays of light pass. The 
 amount of light admitted is regulated by the dilator and con- 
 strictor (sphincter) muscles of the iris, which are able to increase 
 or lessen the size of the pupil ; the iris thus serves as a screen to 
 regulate the amount of light which enters the eye. 
 
 Not only is the size of the pupil inconstant, but the lens is also 
 capable of undergoing considerable change in form, becoming more 
 flattened or more convex, as the case may be. The former con- 
 
 dition occurs when distant, the 
 latter when near objects are 
 looked at. This delicate accom- 
 modating apparatus is regulated 
 by a ciliary muscle (tensor choro- 
 idece) supplied by the oculomotor 
 nerve, which arises in a circle all 
 round the eye from the point of 
 junction of the iris and sclerotic 
 and is inserted along the peri- 
 pheral border of the iris (Fig. 
 168). 
 
 Externally to the vascular 
 layer of the choroid is a lymph- 
 sinus with pigmented walls 
 (lamina fused] ; and externally to 
 this, again, is a firm fibrous, partly 
 
 Op, optic nerve ; OS, sheath of optic cartilaginous, or even ossified 
 nerve ; MF, blind-spot ; Fo, yellow layer, the sclerotic. The latter 
 
 ?$ ( p?gSSu' o r f eti thJ !* internally into the sheath 
 
 retina ; Ch, choroid, with its lamina 01 the OptlC nerve, which is COn- 
 
 FIG. 168. DIAGRAM or A HORIZONTAL 
 SECTION THROUGH THE LEFT 
 HUMAN EYE. (Seen from above.) 
 
 fusca (LJ) and vascular layer (GS) ; 
 jSc, sclerotic ; Co, cornea ; Cj, con- 
 junctiva ; MD, membrane of Desce- 
 met ; OS, canal of Schlemm (the 
 
 tinuous with the dura mater, and 
 externally into the cornea, the 
 outer surface of which is covered 
 
 through the sclerotic to the small 
 oval aperture) ; Ir, iris ; Lc, ciliary 
 ligament ; C, ciliary process ; VK, 
 
 of the eye ; L, lens ; H, hyaloid 
 membrane ; Z, Zone of Zinn, CP, 
 canal of Petit ; Cv, vitreous humour. 
 
 dotted line should extend further over by an epithelial layer con- 
 
 + Vrrf*vn r/l-i + Vm crAa\*f\4-\c* -fr\ 4-l-i ct-rvoll , 1 1 * i 
 
 tinuous with the epidermis the 
 conjunctiva. The- sclerotic and 
 
 HK, anterior and^posterior chamber cornea together form a firm outer 
 
 support for the eye, and thus, to- 
 gether with the gelatinous mass 
 of the vitreous humour, guarantee 
 
 the rigidity necessary for the physiological activity of the nerve 
 end-apparatus. Between the cornea and iris there is a large lymph- 
 space, the anterior chamber of the eye (Fig. 168), its contained 
 fluid being called the aqueous humour. Other lymph-spaces are 
 also present, e.g., between the choroid and sclerotic. 
 
 The deep orbit, formed by the skull, serves as a further pro- 
 tection for the eye, as do also certain accessory structures, which 
 may be divided into three categories, viz. : 
 
EYE 211 
 
 1. Eyelids (palpebrae). 
 
 '2. Glandular organs. 
 
 3. Muscles, serving to move the eye-ball. 
 
 The eye-ball is thus formed of a series of concentric layers 
 which are called from within outwards retina, choroid and iris 
 (vascular layer), and sclerotic and cornea (skeletal layer). The first 
 corresponds with the nervous substance of the brain, the second with 
 the pia mater, and the third with the dura mater. The interior of 
 the eye contains refractive media, the lens and vitreous humour. 
 To these, certain accessory structures are added (pp. 216220). 
 
 The relative development of the eye varies considerably amongst Verte- 
 brates. It may reach a very high degree of perfection ; or may, on the other 
 hand, undergo more or less degeneration in those animals which live in caves 
 or burrows (e.g., Fishes Amblyopsis splelseus, Typhlogobius ; Amphibians 
 Proteus, Gymnophiona ; Snakes Typhlops : Mammals Talpa, &c.). 
 In Ammocoetes and Myxine the eye is hidden beneath the integument (see 
 below), and in the Cetacean Platanista gangetica the eyes are extremely 
 minute. 
 
 The retina will be dealt with after a description of the eyes 
 of the various classes of Vertebrates has been given (p. 214). 
 
 In Amphioxus a simple pigment spot is present in the front wall of the 
 "cerebral ventricle" (p. 157, and Fig. 219). 
 
 Cyclostomes. The eye of Cyclostomes remains at a very low 
 stage of development, not only as regards the structure of the retina, 
 but also in Myxinoids, in the absence of a lens and iris and of a 
 differentiated sclerotic and cornea as well as of eye-muscles, and 
 in the persistence of the choroid fissure. Moreover, the eye in 
 Myxinoids and in the larval Ammoccete lies beneath the skin and 
 subdermal connective tissue. In Petromyzon the skin covering the 
 eye becomes thinned out at metamorphosis, and thus the animal, 
 which was blind, or nearly blind, in the larval state, can see on 
 reaching the adult condition : at the same time the eye becomes 
 more highly organised, though the primary lumen in the lens 
 (Fig. 167, B) does not entirely disappear. 
 
 Fishes and Dipnoans. The eyes of all the true Fishes are, 
 with few exceptions, of considerable relative size, and are formed 
 on essentially the same plan as that described in the intro- 
 ductory portion of this chapter. 
 
 The lens of Fishes, like that of all aquatic animals, is globular, 
 and possesses therefore a high refractive index. It touches the 
 cornea and fills up the greater part of the eyeball, so that only a 
 small space is left for the vitreous humour. It differs from that of 
 other Vertebrates in the fact that, in the condition of rest, it is 
 accommodated for seeing near objects. In Teleosts accommodation 
 apparently takes place by means of a process of the choroid, the 
 processus falciformis. This extends into the vitreous humour 
 towards the lens, around which it expands to form the so-called 
 
 p 2 
 
212 
 
 COMPARATIVE ANATOMY 
 
 VK Co 
 
 campanula Halleri (Fig. 169). In the interior of this structure 
 are nerves, vessels, and smooth muscle-fibres, and the latter possibly 
 
 exert an influence on the lens, draw- 
 ing it towards the retina. The pro- 
 cessus falciformis is never large in 
 Ganoids and is absent in Cyclo- 
 stomes, Elasmobranchs, and Dip- 
 noans : the question of accommoda- 
 tion in these Fishes is not under- 
 stood. 
 
 Externally to the choroid proper, 
 that is, between it and the lamina 
 fusca, lies a silvery or greenish-gold 
 iridescent membrane, the argentea. 
 It extends either over the whole 
 interior of the eye (Teleosts), or is 
 limited to the iris (Elasmobranchs). 
 A second layer with a metallic 
 lustre, the tapetum lucidum, is 
 present internally to the iridescent 
 portion, and within this again is the 
 chorio-capillaris of the choroid. No 
 tapetum appears to be present in. 
 Teleostei or Petrornyzon. 
 
 The so-called choroid gland, pre- 
 sent only in Teleostei and Amia, 
 consists of a network of blood- 
 vessels (rete mirabile) which has the form of a cushion, lying near 
 the entrance of the optic nerve, between the argentea and pigment 
 epithelium of the retina : it thus has nothing to do with a " gland." 
 The sclerotic is usually extensively chondrified, and not unfre- 
 quently becomes calcified or ossified towards its junction with the 
 cornea. 
 
 The eyeball is almost always surrounded by a gelatinous tissue, penetrated 
 by connective-tissue fibres, and in Elasmobranchs it is usually articulated on its 
 inner circumference with a rod of cartilage connected distally with the 
 lateral wall of the skull. 
 
 Amphibia. The eyes of Amphibians are proportionately 
 smaller, and their form rounder than those of Fishes, but there are 
 many points of close correspondence between them. This is true, 
 for instance, as regards the more or less distinctly chondrified 
 sclerotic, the slightly convex cornea, and the globular lens. In other 
 important respects, however, the Amphibian eye is simpler than 
 that of Fishes ; thus it is wanting in an argentea, a tapetum, a 
 choroid gland, and a processus falciformis and campanula Halleri. 
 The iris contains smooth muscle-fibres, and a true ciliary muscle 
 is present in the whole series of animals from this point onwards, 
 though not strongly developed in Amphibians. The pupil is usually 
 round, but may be angular. 
 
 FIG. 169. EYE OF A TELEOSTEAN. 
 
 Op, optic nerve ; OS, sheath of optic 
 nerve ; Rt, retina ; PE, pigment 
 epithelium ; Tp, tapetum ; Lr, 
 lamina vasculosa ; Ay, argentea ; 
 Ls, lamina supra-choroidea ; Sc, 
 sclerotic, enclosing cartilage or 
 bone (f) ; Co, cornea ; Ir, Iris ; 
 VK, anterior chamber ; L, Lens ; 
 CV, vitreous humour ; Pr, pro- 
 cessus falciformis ; Cp, campanula 
 Halleri. 
 
EYE 
 
 213 
 
 FIG. 170. EYE 
 OF Lacerta mu- 
 rcdis, SHOWING 
 THE RING OF 
 BONY SCLERO- 
 TIC PLATES. 
 
 The eyes of Proteus and of the Gymnophiona, as already mentioned, 
 always lie more or less deeply beneath the skin ; they are very small, and 
 are much degenerated. In Proteus the crystalline lens and iris are both 
 wanting, and the vitreous humour is only slightly developed. 
 
 Reptiles and Birds. In these also, the sclerotic is in great 
 part cartilaginous, and in Lizards and Chelonians it is provided 
 with a ring of delicate bony sclerotic plates around 
 the external portion (Fig. 170). Many fossil Rep- 
 tiles and Amphibians possessed similar plates, as 
 do also existing Birds (Fig. 171) ; in Birds horse- 
 shoe- or ring-shaped bony structures are also 
 usually present close to the entrance of the optic 
 nerve. 
 
 The eyeball of Reptiles has a globular form 
 (Fig. 170), while that of Birds, more especially 
 nocturnal Birds of prey (Owls), is more elongated 
 and tubular, an external larger segment being 
 sharply marked off from an internal smaller 
 one : moreover the whole eye is relatively larger. 
 
 (Fig. 171). The 
 
 outer portion is bounded ex- 
 ternally by the very convex 
 cornea and encloses a large 
 anterior chamber as well as a 
 complicated ciliary muscle com- 
 posed of striated fibres. This 
 muscle is also transversely stri- 
 ated in Reptiles, in which 
 especially in Chelonians, it is 
 always well developed, though 
 not to such an extreme degree 
 as in Birds. 
 
 In Reptiles (Lizards, for in- 
 stance) a tapetum may be 
 developed, but an argentea and 
 choroid gland are never present; 
 all these parts are wanting in 
 Birds. A structure which is 
 FIG. 171. EYE OF AN OWL. homologous with the processus 
 
 falciformis of Fishes is, how- 
 ever, present in most Reptiles 
 and in Birds. Absent in 
 Hatteria and the Chelonia, 
 this so-called pecten is largely 
 developed in Birds x (Fig. 171), 
 and may extend from the point 
 of entrance of the optic nerve 
 to the capsule of the lens, but 
 1 In Apteryx the pecten disappears during development. 
 
 Rt, retina ; Ch, Choroid ; Sc, sclerotic, 
 with its bony ring at t ; CM, ciliary 
 muscle ; Co, cornea ; VN, point of 
 junction between sclerotic and cornea ; 
 Ir, iris ; VK, anterior chamber ; L, 
 lens ; O, vitreous humour ; P, pecten ; 
 O/J, OS, -optic nerve and sheath. The 
 dotted line passing across the broadest 
 portion of the circumference of the eye 
 divides the latter into an inner and 
 an outer segment. 
 
214 COMPARATIVE ANATOMY 
 
 as a rule does not reach so far. In Birds it is always more or 
 less folded, and consists rnainly of a closely-felted network of 
 capillaries. In both Reptiles and Birds, the pecten appears to 
 be important in the nutrition of the contents of the eyeball 
 and of the retina : it has nothing to do with accommodation. 
 
 The iris, which is regulated by striated muscle, by means of which it is 
 able to respond very quickly to visual impressions, is often brightly coloured, 
 and this colour is due to the presence, not only of pigment, but also of 
 coloured fat globules. 
 
 The pupil is as a rule round, but in many Reptiles and in Owls has the 
 form of a vertical slit. 
 
 Mammals. In Mammals the eyeball is always more com- 
 pletely enclosed within the bony orbit than is the case in most 
 other Vertebrates, and this may partially account for the fact that, 
 except in Monotremes, the sclerotic no longer shows traces of 
 cartilage or bone, but is entirely of a fibrous character (Fig. 168), 
 
 With the exception of aquatic Mammals, in which it is some- 
 what flattened, the cornea is moderately convex, and the whole 
 eyeball is of a more or less rounded form. 
 
 A tapetum lucidum, consisting either of cells or fibres, exists in the choroid 
 of numerous Mammals, and gives rise by interference to a glistening appear- 
 ance when seen in the dark (Carnivores, Ruminants, Perissodactyles, &c. ). 
 
 Certain structures homologous with the processus falciformis and pecten. 
 are present in Mammals in the embryo only. 
 
 The ciliary muscle consists of smooth elements. 
 
 The external surface of the lens is less convex than the internal, which 
 latter lies in the so-called fossa patellaris of the vitreous humour. 
 
 The pupil is not always round, but may be transversely oval (Ungulates, 
 Kangaroos, Cetaceans), or slit-like and vertical (e.g., Cat). 
 
 Retina. 
 
 The fibres of the optic nerve, which pass into the eyeball at a 
 right or acute angle, cross one another at the point of entrance, 
 and are then distributed to the sensitive elements of the retina. 
 The latter is thus thickest at the point of entrance of the nerve, 
 which is known as the "blind spot" (Fig. 168), and gradually de- 
 creases in thickness towards the ciliary processes, until, at the point 
 of origin of the iris, it consists of a single layer of cells. 
 
 The retina is bounded externally by a structureless hyaline 
 membrane (limitans externa), 1 while on its inner side it is covered 
 by the hyaloid membrane, which, strictly speaking, belongs 
 to the vitreous humour. The retina is quite transparent in the 
 fresh condition, and consists of two portions which are histo- 
 logically and physiologically quite distinct: they are, a supporting 
 
 1 The membrana limitans encloses the entire retina externally in the embryo, 
 but later the rods and cones come to project through it (see Fig. 172). 
 
RETINA 
 
 215 
 
 part and a nervous part. The former is stretched as on a frame 
 between the limitans externa and hyaloid membrane. 
 
 The nervous elements are arranged in the following concentric 
 layers : 
 
 I. Developed from the internal layer of the, secondary optic vesicle. 
 
 A. Cerebral layer. 
 
 1. Layer of nerve-fibres (of optic nerve). 
 
 2. Layer of ganglion-cells. 
 
 3. Inner reticular layer. 
 
 4. Granular layer (inner). 
 
 5. Outer reticular or subepithelial layer. 
 
 B. Epithelial layer. 
 
 6. Layer of visual cells (outer granular layer with the 
 
 rods and cones). 
 
 II. Developed from, the external layer of the secondary optic vesicle. 
 
 7. Pigment epithelium (retinal epithelium). 
 
 It seems probable that the various nerve-cells of the retina are not directly 
 connected with one another, but are only contiguous. 
 
 Rods and cones, 
 
 Memhrana 
 
 limitans. 
 
 Outer granular 
 layer. 
 
 Concentric 
 
 supporting-cells 
 
 (nucleated). 
 
 Concentric 
 supporting-cells 
 (non-nucleated). 
 
 Radial fibres. 
 
 Radial fibres. ~ 
 
 Outer recticular 
 
 layer. 
 
 Sub-epithelial 
 ganglion-cell. 
 
 Star-shaped 
 ganglion-cell. 
 
 Bipolar 
 ganglion-cell. 
 Multipolar 
 ganglion-cell. 
 Inner recticular 
 layer. 
 
 Centrifugal 
 nerve fibres. 
 
 Multipolar 
 ganglion-cell. 
 
 Layer of 
 nerve fibres. 
 
 FIG. 172. DIAGRAM OF THE ELEMENTS OF THE RETINA. (Supporting elements 
 on the left, and nervous elements on the right. ) After Ph. Stohr. 
 
 These layers are so arranged that the nerve-fibres lie next to 
 the vitreous humour, that is, internally, while the rods and cones 
 
216 COMPARATIVE ANATOMY 
 
 are situated towards the choroid, or are external. Thus the 
 terminal elements of the neuro-epithelium are turned away 
 from the rays of light falling upon the retina, and the rays must 
 therefore pass through all the other layers before they reach the 
 rods and cones. 
 
 Fishes possess the longest, Amphibians the thickest rods, so that in the 
 latter there are only about 30,000 to a square millimetre, while in Man there 
 are from 250,000 to 1,000,000. 
 
 In Fishes the rods far exceed the cones in number, while in Reptiles and 
 Birds the reverse is the case. The cones of many Reptiles and all Birds are 
 distinguished by the presence of brightly coloured oil-globules, which are 
 also present in those of Marsupials. 
 
 In the centre of the retina of higher Vertebrates there is a specially 
 modified region of most acute vision, called the yellow-spot (fovea 
 centralis or macula lutea). It is due to the thinning-out of all the 
 layers except that of the rods and cones, and even the rods disappear, only 
 the cones persisting (Fig. 168). 
 
 Accessory Organs in Connection with the Eye. 
 
 (a) EYE-MUSCLES. 
 
 The movement of the eyeball is always (except in Myxinoids, 
 comp. p. 211) effected by six muscles, four of which are known 
 as the recti (superior, inferior, anterior or internal, and posterior or 
 external), and two as the dbliqui (superior and inferior). The 
 former, which arise from the inner portion of the orbit, usually 
 from the dural sheath of the optic nerve, together circumscribe a 
 pyramidal cavity, the apex of which lies against the inner portion 
 of the orbit, while the base surrounds the equator of the eyeball, 
 where the muscles are inserted into the sclerotic. 
 
 Both the oblique muscles usually arise from the anterior or 
 nasal side of the orbit, and as they respectively pass from this region 
 dorsally and ventrally in an equatorial direction round the eyeball, 
 they constitute a sort of incomplete muscular ring. 
 
 A deviation from this arrangement is seen in Mammals, in which the 
 superior oblique has gradually come to arise from the inner part of the 
 orbit, and then passes forwards towards its anterior (internal) angle, where 
 it becomes tendinous, and passes through a fibre-cartilaginous pulley (trochlea) 
 attached to the upper border of the orbit, on the frontal bone. Hence it is 
 sometimes called the trochlear muscle. From this point it changes its direc- 
 tion, and becomes reflected obliquely outwards and backwards to the globe of 
 the eye. 
 
 Besides these six muscles, others are usually present which 
 are known as the retractor lulli (best developed in Ungu- 
 lates), the quadratus (bursatis), and the pyramidalis. The last 
 two are connected with the nictitating membrane (see p. 217), 
 and are present in Reptiles and Birds. All three are supplied by 
 the abducent nerve (comp. p. 184). 
 
GLANDS 217 
 
 (6) EYELIDS 
 
 In Fishes and other lower aquatic forms the upper and lower 
 eyelids are usually very rudimentary, having at most (e.g., Elasmo- 
 branchs) the form of stiff folds of the skin ; and in all Verte- 
 brates below the Mammalia they never reach a very high stage of 
 development. They are lined on the surface looking towards the 
 eyeball by a continuation of the epidermis, the conjunctiva (p. 210), 
 and in the Ichthyopsida and Sauropsida are usually not sharply 
 marked off from the rest of the skin, being capable of no, or only 
 of very slight, movement. 1 
 
 In Mammals, the eyelids, more particularly the upper one, are 
 extremely movable, and are provided with hairs (eyelashes) on 
 their free margin. In their interior a hard body, the so-called 
 " lid-cartilage " is developed, and they are closed by a circular 
 muscle which surrounds the whole slit between the lids ; a levator 
 is also present in the upper eyelid. In Sauropsida and many 
 Mammalia (e.g., Ungulates) there is a depressor of the lower lid. 
 
 The want, or comparatively slight development of upper and 
 lower eyelids in Vertebrates below the Mammalia is compen- 
 sated for in certain forms, at any rate to a certain extent, by the 
 presence of a nictitating membrane. This " third eyelid " differs 
 from the others in having nothing to do with the outer skin proper, 
 consisting simply of a reduplicature of the conjunctiva, and being 
 regulated by special muscles (see p. 216). 
 
 The nictitating membrane, which is represented in certain 
 Elasmobranchs (e.g., Carcharias, Galeus, Zygsena, Mustelus, comp. 
 p. 143) and which often encloses a cartilage, is situated within 
 the lower eyelid, or it may lie more towards the anterior angle 
 of the eye. The former condition is seen, e.g., in Anurans, and 
 the latter in Birds, in which a third eyelid is so largely developed 
 as to be capable of covering the whole freely exposed portion of 
 the eyeball. In Reptiles and Mammals it always lies in the 
 anterior angle of the eye ; in Primates it becomes reduced to a 
 small, half-moon-shaped fold (plica semilunaris), but in Monkeys 
 and certain races of Mankind traces of the cartilage are present. 
 
 (c) GLANDS. 
 
 The glands in connection with the eye are : (1) the lachrymal, 
 (2) the Harderian, or gland of the nictitating membrane, and (3) 
 the Meibomian glands. 
 
 The secretions of all these serve to keep the free surface of the 
 eyeball moist, and to wash away foreign bodies. In Fishes and 
 
 1 In many Reptiles and Birds the upper eyelid, is supported by a membrane- 
 bone or fibre-cartilage. In Geckos, Amphibsenians and Snakes the two eyelids 
 grow together to form a transparent membrane overlying the eye, and this comes 
 away with the rest of the outer part of the skin when the latter is shed. 
 
218 
 
 COMPARATIVE ANATOMY 
 
 FIG. 173. HARDERIAN 
 GLAND (H, H l ) AND 
 LACHRYMAL GLAND (Th) 
 OF Anguis fragilis. 
 
 M, muscle of jaw ; B, eye- 
 ball. 
 
 Dipnoans, 1 the outer medium appears to suffice for this purpose, 
 
 but the first attempt of a Vertebrate to exchange an aquatic for an 
 aerial existence necessitated the develop- 
 ment of a secretory apparatus in connection 
 with the eye. 
 
 Thus in Urodeles a glandular organ is 
 developed from the conjunctival epithelium 
 along the whole length of the lower eye- 
 lid ; in Reptiles this becomes more 
 developed in the region of the anterior 
 and posterior angles of the eye, and the 
 original connecting bridge gradually dis- 
 appears : thus two glands are developed 
 from the primitively single one, each of 
 which becomes further differentiated both 
 histologically and physiologically. From 
 
 one is formed the Harderian gland, which always lies at the 
 
 anterior angle of the eye, sur- 
 
 rounding to a greater or less ex- 
 
 tent the antero-ventral portion of 
 
 the eyeball, while the other gives 
 
 rise to the lachrymal gland 2 
 
 (Figs. 173 and 175). The latter 
 
 retains throughout life its primi- 
 
 tive position at the posterior 
 
 angle of the eye, and even 
 
 in Birds lies in the region of 
 
 the lower eyelid ; it is supplied 
 
 by the second division of the 
 
 trigeminal. In Mammals it be- 
 
 comes gradually further sub- 
 
 divided, and extends into the 
 
 region of the upper eyelid, so 
 
 that its ducts open above the 
 
 eye into the upper conjunctival 
 
 sac (Fig. 175, A & B). Never- 
 
 theless, even in the ,, Primates, 
 
 more or fewer ducts are present 
 
 which open into the lower con- 
 
 lit 
 
 Fro. 174. DIAGRAMMATIC TRANSVERSE 
 VERTICAL SECTION THROUGH THE 
 EYE OF A MAMMAL. 
 
 junctival sac, and thus the primi- 
 tive position of the lachrymal 
 gland is indicated. 
 
 A well-differentiated Har- 
 derian gland is present from the 
 
 tailless Amphibia to the Mammalia, but is very rudimentary in 
 the Primates. 
 
 Op, optic nerve ; B, eyeball ; Fo, Fo, 
 upper and lower conjunctival sac ; 
 LH, LH, outer skin of the eyelids, 
 which at the free edges of the" latter 
 at t becomes continuous with the 
 conjunctiva ; T, the so-called tarsal 
 fibre-cartilages, in which the Meibo- 
 mian glands (MD) lie embedded, 
 the latter opening at * ; //, H, eye- 
 lashes. 
 
 1 Comp. p. 17. 
 
 - A lachrymal gland is absent in Crocodiles and Snakes. 
 
GLANDS 
 
 219 
 
 The Meibomian ylands, belonging to the group of sebaceous 
 glands, are confined to the Mammalia, and lie embedded in the 
 
 FIG. 17oA. DIAGRAM TO ILLUSTRATE THE SHIFTING OF THE LACHRYMAL GLAND 
 
 WHICH HAS TAKEN PLACE IN THE COURSE OF PHYLOGENY. 
 
 The gland shifts in the direction of the arrows ; a, its position in the Amphibia ; 
 b, in Reptiles and Birds, and occasionally in Man, in which case it may be 
 regarded as atavistic ; c, normal position in Man. 
 
 substance of the eyelids in the form of branched tree-like tubes 
 or clustered masses. They open on the free edge of the lid, and 
 produce a fatty secretion. Certain modified sweat-glands known as 
 
 FIG. HOB. DIAGRAM OF THE LACHRYMAL APPARATUS OF MAN. 
 
 TD, lachrymal gland, divided up into several portions ; **, ducts of the lachrymal 
 
 fland ; tt, puncta lachrymalia ; TR, TR 1 , upper and lower lachrymal canals; 
 , lachrymal sac ; D, naso-lachrymal duct. 
 
 the glands of Moll are also present immediately within the eyelids 
 of Mammals. 
 
 The naso-lachrymal duct, which conducts the lachrymal 
 secretion into the nose, has already been referred to (p. 201). 
 
220 
 
 COMPARATIVE ANATOMY 
 
 In the Cetacea, the lachrymal and Meibomian glands, as well as the naso- 
 lachrymal duct, are wanting, and a lachrymal duct is absent in the Otter, Seal, 
 and Hippopotamus. In the two last-mentioned animals the lachrymal gland 
 is much reduced : in Manis javanica there are no Meibomian glands, and in 
 the Mole the entire lachrymal apparatus has undergone reduction. 
 
 AUDITOKY ORGAN. 
 
 It is very probable that the auditory organ, like the organs 
 of smell and taste, has been derived primitively from a modified 
 integumentary sense-organ. It is developed from an invagination 
 
 FIG. 176. HEAD AND ANTERIOR 
 PORTION OF BODY or A CHICK. 
 . (In part after Moldenhauer. ) 
 
 EG, olfactory pit ; A, eye ; / to 
 IV, first to fourth visceral 
 arches ; t, point at which the 
 external auditory passage begins 
 to be formed ; LB, primitive 
 auditory vesicle seen through 
 the wall of the head. 
 
 se 
 
 ass 
 
 FIG. 177. SEMIDIAGRAMMATIC FIGURE OF 
 THE MEMBRANOUS LABYRINTH OF VERTE- 
 BRATES. (Seen from the outer side.) 
 
 11, utriculus ; rec, recessus utriculi ; up, sinus 
 posterior utriculi ; s, sacculus ; /, recessus 
 sacculi (lagena) ; cus, utriculo-saccular 
 canal ; de, se, ductus and saccus endolym- 
 phaticus, the former arising from the 
 sacculus at f ; ss, sinus utriculi superior ; 
 ass, apex of the same ; ca, ce, cp, anterior, 
 external, and posterior semicircular canals; 
 aa, ae, ap, the corresponding ampullae. 
 
 of the ectoderm on either side of the primary hind-brain : this be- 
 comes separated off to form a vesicle (-Fig. 176), and its epithelium 
 is differentiated into elongated cells of sensory epithelium pro- 
 vided with hair-like processes (Figs. 178 A and B) separated by 
 supporting cells. The sensory cells are surrounded by a nerve- 
 network, and are not continuous with the nerves as in the case of 
 the olfactory cells (p. 197). 
 
 Like the other higher sense-organs, the paired auditory organ 
 
AUDITORY ORGAN 
 
 221 
 
 of Vertebrates is situated in the region of the head, between the 
 origins of the trigeminal and vagus nerves. After the vesicle of 
 each side has become separated off from the epiblast and connected 
 with the brain by means of the auditory nerve (which arises in 
 connection with a peripheral ectodermic ganglion and then grows 
 centripetally to the brain), it sinks deeper and deeper into the 
 mesoblastic tissue of the skull : it then loses its original pyriform 
 or rounded shape, and becomes divided into two parts, called re- 
 spectively the utriculus .and sacculus (Fig. 177). From the former 
 
 FiC4. 178A. ISOLATED ELEMENTS OF THE MEMBRANOUS LABYRINTH OF VARIOUS 
 VERTEBRATES. (After G. Retzius. ) 
 
 A, from the macula acustica communis of Myxine glutinosa ; B, from the macula 
 acustica neglecta of Raia davata ; C, from the crista acustica of an ampulla 
 of Linedon (Amblystoma) mexicanu* ; D, from the crista acustica of the 
 anterior ampulla of Roma esculanta. 
 
 hz, hair-cells with auditory hairs (h) ; fz, thread-like cells; n,n, dividing nerve. 
 On the left side of D the auditory hair has become broken up into its con- 
 stituent fibres. 
 
 the semicircular canals become developed, while from the latter 
 the tube-like ductus endolympJiaticus and the lagena (cochlea) are 
 formed. 
 
 The whole of this complicated apparatus constitutes the internal 
 ear or membranous labyrinth. It becomes surrounded secondarily 
 by mesoblastic tissue, with which it is at first in close contact. 
 A process of absorption then takes place in the innermost layers 
 of the mesoblast, and thus a space is developed which closely 
 
COMPARATIVE ANATOMY 
 
 repeats the form of the membranous labyrinth, as does also the 
 mesoblast which encloses this space and which later becomes 
 chondrified, and often also ossified. A membranous and a bony laby- 
 rinth can thus be distinguished, and between them is a cavity 
 (cavum perilymphaticwri) filled with a lymph-like fluid {perilympK). 
 The cavity within the membranous labyrinth, which also contains a 
 fluid (endolymph), is spoken of as the cavum endolymphaticum. 
 
 Except in Cyclostomes, three semicircular canals are always 
 present, and these lie in planes at right angles to one another. 
 They are distinguished as the anterior vertical, the posterior 
 vertical, and the horizontal (external) canals (Fig. 177). The first 
 and last-named arise from the portion of the utriculus known as 
 the recessus utricuii, and each has a vesicle-like swelling or ampulla 
 
 \ a 
 
 FIG. 178B. LONGITUDINAL SECTION OF AN AMPULLA OF GOBIUS. (The exact form 
 of the epithelium of the crista is not indicated.) After Hensen. 
 
 11, the nerve passing into the connective-tissue of the crista; a, base of semi- 
 circular canal ; b, point of opening of the ampulla into the utriculus ; c, the 
 epithelium on the free wall of the ampulla ; d, the auditory hairs. 
 
 at its origin. The posterior canal also arises with an ampulla from 
 a prolongation of the utriculus (sinus posterior). The other end of 
 the horizontal canal opens by a funnel-shaped enlargement into the 
 utriculus, while that of the anterior and of the posterior canal 
 fuse together to forma common tube, the so-called canal commissure 
 (sinus superior) , which also opens into the utriculus. 
 
 Concretions composed mainly of carbonate of lime are present 
 in the regions of the various nerve end-plates of the auditory 
 organ in all Vertebrates. These otoliths present the greatest 
 variety both in form and size. The largest and most massive ones 
 are seen in Teleosts. They either consist of a single mass, or are 
 arranged in groups in different regions of the labyrinth. 
 
AUDITORY ORGAN 
 
 223 
 
 The sensory epithelium, to which the branches of the auditory 
 nerve are distributed, is situated in the following parts of the 
 membranous labyrinth : (1) the three ampullae of the canals, in 
 each of which the auditory cells are situated on a ridge (crista 
 asustica) projecting into the lumen (Fig. 178s) ; (2) a large macula 
 acustica in the utriculus : this is continued into the recessus utricuJi 
 
 FIG. 179. DIAGRAM OF THE ENTIRE AUDITORY ORGAN OF MAN. 
 
 External Ear. M, M, pinna ; Mae, external auditory meatus ; O, wall of latter; 
 Mt, tympanic membrane. 
 
 Middle Ear. Ct, Ct, typmanic cavity ; O 1 , wall of same ;4p sound-conducting 
 apparatus, indicated by a rod, representing the auditory ossicles, the end of 
 the rod marked t corresponds to the stapes, which closes up the feuestra 
 ovalis ; M, fenestra rotunda ; Tb, Eustachian tube ; Tb l , its opening into the 
 pharynx ; 0", its wall. 
 
 Internal Ear, with the greater part of the bony labyrinth (KL, KL 1 ) removed. 
 S, sacculus ; a, b, the two vertical canals, one of which (b) is shown cut 
 through ; c, Co, commissure of the canals of the membranous and bony laby- 
 rinths respectively ; S.e, D.e, saccus and ductus endolymphaticus ; the latter 
 bifurcates at 2 ; Op, cavum perilymphaticum ; Cr, canalis reunions ; Con, 
 membranous cochlea, which gives rise to a blind sac at -f- ; Con 1 , bony cochlea ; 
 Sv and St, scala vestibuli and scala tympani, which at * pass into one another 
 at the cupula terminalis (Ct) ; D.p, ductus perilymphaticus, which arises 
 from the scala tympani at d, and opens at D.p 1 . The horizontal canal is 
 seen between 2 and S. 
 
 as well as into the sacculus and lagena, or rudiment of the cochlea, 
 which arises from the sacculus ; (3) the rudimentary macula 
 acustica ncglccta, which in Fishes, Birds, and Reptiles is situated on 
 the floor of the utriculus close to the sacculo-utricular canal. In 
 Amphibians it lies on the inner side of the sacculus, and in 
 Mammals undergoes a gradual reduction and may even become 
 
224 COMPARATIVE ANATOMY 
 
 obliterated. The several portions of the sensory plate or macula 
 acustica, which are originally continuous, become later disconnected 
 from one another, and except in Cyclostomes are seen as separate 
 maculae acusticse. 
 
 The higher we pass in the Vertebrate series, the greater share 
 Joes the mesoblast take in the formation of the auditory organ. 
 At first that is, in Fishes the membranous labyrinth or internal 
 ear lies close under the roof of the skull, and is thus easily 
 accessible to the waves of sound, which are conducted partly 
 through the operculum (when present), and partly through the 
 gill-slits or spiracle. As we pass to the higher animals, however, 
 the auditory organ gradually sinks further and further inwards from 
 the surface, so that a new method for conducting the sound-waves 
 becomes necessary, and certain accessory structures are developed 
 (Fig. 179). A canal, the external auditory passage or meatus, passes 
 .inwards from the surface ; this opens into a spacious chamber, the 
 tympanic cavity, in which are situated the auditory ossicles, and 
 which is connected by the Eustacliian tube with the pharynx. The 
 whole of this canal, which is divided into outer and inner portions 
 (external and middle ear) at the junction of the external auditory 
 passage and tympanic cavity by a vibratory membrane, the tympanic 
 membrane, lies in the position of the first embryonic visceral 
 (hyoid or spiracular) cleft. From Reptiles and Birds onwards the 
 first indications of a pinna (that is, the part of the external ear 
 which projects from the head) are seen, but this only reaches a 
 full development in Mammals. 
 
 Cyclostomes. In Petromyzon there are only two (the vertical) 
 semicircular canals, and in Myxine only one canal is present, which, 
 as it possesses two ampul Ia3, probably represents the two fused 
 together (Fig. 180A). 
 
 Fishes and Dipnoans, The auditory organ of all the true 
 Fishes (Fig. ISOA^c) follows the general plan given above, and the 
 same may be said for all higher Vertebrates. Almost without 
 exception we meet with a division into a pars superior represented 
 by the utriculus and semicircular canals, which remains essentially 
 much in the condition already described, and a pars inferior 
 constituted by the sacculus and lagena, which gradually becomes 
 more differentiated, and attains to a higher and higher degree of 
 development and functional perfection. In Fishes the lagena 
 consists simply of a small knob-like appendage of the sacculus, 
 which opens freely into the main cavity of the latter by means 
 of the sacculo-cochlear canal : it is absent in Chimsera. The 
 utriculus and sacculus also communicate with one another by 
 the sacculo-utricular canal. In Elasmobranchs the ductus endo- 
 lymphaticus opens dorrally on the posterior part of the head, and is 
 thus in free communication with the sea- water. 
 
-ha. 
 
 Fio. 180 MEMBRANOUS LABYRINTH OF VARIOUS FISHES (after G. Retzius). 
 A, Myxim glutinosa, from the inner side. 
 
 se, saccus communis ; aa, ap, anterior and posterior ampulla.; cc, canalis com- 
 munis ; de, ductus endolymphaticus ; se, sacons endolymphaticus ; me, macula 
 acustica communis ; era, crista acustica of the anterior, and crp, of the 
 posterior ampulla ; ra, rp, anterior and posterior branches of the auditory 
 nerve. 
 
 A 1 , Aeimnser sturio, outer side ; B, Chimcera monttrosa, inner side ; C, Perca 
 Jlumatilis, inner side. 
 
 it, utriculus ; ss, sp, sinus utriculi superior and posterior ; o-vs, apex of the sinus 
 superior ; rec, recessus utriculi ; aa, ae, ap, anterior, external, and posterior 
 ampulla ; eft, cp, ee, anterior, posterior, and horizontal (external) semicircular 
 canals ; ., sacculus ; ens, utriculo-saccular canal ; de, ductus endolymphati- 
 cus, which in B opens externally through the skin ha at ode ; se, saccus 
 endolymphaticus ; /, lagena ; mn, macula acustica of the recessus utriculi ; 
 er, crista acustica of the ampullae ; ms. macula acustica of the sacculus ; mn, 
 macula acustica neglecta ; pi, papilla acustica of the lagena ; ac, auditory 
 nerve ; raa, rae, rap, ru, rs, rl, m, the various branches of the same ; o 
 (in C), otoliths (in the recessus utriculi, sacculus, and lagena, ) 
 
 Q 
 
226 COMPARATIVE ANATOMY 
 
 In Chimseroids, Ganoids, Teleosts and Dipnoans, the auditory 
 capsules are not completely surrounded by cartilage or bone, the 
 perilymphatic and cranial cavities only being separated by a fibrous 
 partition. 
 
 In certain Teleosts (Siluroidei, Gymnotidse, Characinidre, Gymnarchidra, 
 Cyprinoidae) the auditory organ comes into relation with the air-bladder by 
 means of a chain of bones (" Weberian ossicles") derived from certain parts 
 of the four anterior vertebrae and corresponding pairs of ribs, and by this 
 means the relative fulness of the air-bladder can be appreciated by the Fish. 
 Connections between processes of the air-bladder and the internal ear are 
 also met with in several other Teleosts. 
 
 The auditory organ of. the Dipnoi most nearly resembles that 
 of Elasmobranchii, and more particularly that of Chimsera. 
 
 Amphibia. The membranous labyrinth of Amphibians re- 
 sembles that of Fishes and Dipnoans in many respects, but impor- 
 tant differences are seen, more particularly as regards the lagena, 
 which, especially in the Anura, becomes further constricted off 
 from the sacculus and reaches a higher stage of development. 
 Traces of a papilla acustica lagenye lying within the lagena are met 
 with in the Myctodera, and even in Menopoma and Siredon. In 
 the Anura (Fig. 181) a higher condition is seen in the presence of 
 a small ridge-like outgrowth in the interior of the thickened lagena 
 on which a definite region, supported by cartilage, corresponds to 
 the basilar membrane of higher types ; this bears another patch of 
 nerve endings the papilla acustica basilaris. 
 
 The ductus endolymphaticus, as in certain Teleosts, may give 
 rise to large sac-like enlargements containing calcareous matter 
 and lie close to its fellow, either on the dorsal surface only, or 
 on both dorsal and ventral sides of the brain. The latter is the 
 case in Anura, for instance, in which the sac extends as an unpaired 
 structure along the whole vertebral canal dorsally to the spinal cord, 
 giving rise to paired outgrowths extending through the inter- 
 vertebral foramina and forming the characteristic calcareous bodies 
 situated close to the spinal ganglia. These are lined by pavement 
 epithelium and are plentifully supplied with capillaries : they are 
 riot glandular, as was formerly supposed. 
 
 A further advance in structure as compared with Fishes is seen 
 in the gradual differentiation of a middle ear. In the outer wall 
 of the auditory capsule is a membranous space, the fenestra ovalis, 
 which is plugged by a cartilaginous stapcdial plate ; and from the 
 latter a rod-like cartilage or bone, the columella, usually extends 
 outwards towards the quadrate (p. 84). A tympanic cavity, with 
 a tympanic membrane supported by a ring of cartilage lying on 
 the level of the skin, and a HJustackian tube opening into the 
 pharynx and corresponding phylogenetically to the hyoid cleft 
 of Fishes, are met with in most Anura, in which also the colu- 
 mella is more perfect, consisting of a bony and cartilaginous rod 
 
AUDITORY ORGAN 227 
 
 expanded distally to fit against the tympanic membrane. The 
 colurnella is wanting in certain Urodeles (e.g., Triton). A mem- 
 branous fenestra rotunda in the outer wall of the auditory capsule 
 is present in most Amphibians and in all higher Vertebrates in 
 addition to the fenestra ovalis. 
 
 The ear of the Gymiiophioiia resembles that of the Urodela, but the 
 membranous labyrinth shows further complications. 
 
 FIG. 181. RIGHT MEMBRANOUS LABYRINTH OF Rana esculenta, from the inner 
 side. (After G. Retzius. ) 
 
 <JLU, aperture of utriculus ; I, lagena cochlea? ; pb, pars basilaris cochleae ; CMS, 
 utriculo-saccular canal ; mu, ms, mn, macula acustica recessus utriculi, 
 sacculi, and iieglecta ; pi, ppb, papilla acustica lagense and basilaris ; rua, rap, 
 rs, rn, rl, rb, branches of auditory nerve to the anterior and posterior ampulla?, 
 sacculus, macula neglecta, lagena, and pars basilaris. (Other letters as in 
 Fig. 180, A 1 C.) 
 
 Reptiles and Birds. In Chelonians, the auditory organ 
 shows many points of resemblance to that of Urodeles ; and in all 
 Reptiles and Birds the chief modifications are confined to the 
 lagena or cochlea, which gradually shows a higher condition of 
 development in passing from Chelonians and Snakes to Lizards 
 and Crocodiles. In Chelonians, the cochlea grows out in the form 
 of a short canal ; in Crocodiles arid Birds this canal is considerably 
 longer, and at the same time it becomes slightly coiled (Figs. 182 
 and 183). A more marked differentiation also gradually takes 
 place in the membrana basilaris and the papilla acustica basilaris. 
 Both become more and more elongated ; and, at the same time, 
 distinct indications of scalce tympani and vestibuli are seen. (Com- 
 pare the description of these parts on p. 232.) 
 
 In the Lacertilia the most varied types of auditory organ are met with ; 
 in many (Phrynosoma, Pseudopus, Anguis), the membrana basilaris is hardly 
 more highly developed than in Ophidia. In Iguana, an advance towards 
 Lacerta and the other higher Lizards is to be noticed : the membrana 
 basilaris is longer, though the lagena with its papilla is not so prominent. 
 In Acantias and Platydactylus this state of things is carried still further, and 
 finally the more highly developed auditory ororan of Plestiodon and Egerina 
 
 Q 2 
 
228 
 
 COMPARATIVE ANATOMY 
 
 leads up to that of Crocodilia. Thus there is a continuous and an unbroken, 
 series from the lower forms to the higher. 
 
 The structure of the auditory organ of Hatteria shows many striking, 
 peculiarities : it thus, like that of Chainreleo, occupies an isolated position. 
 
 While the cochlea gradually becomes more independent of the 
 sacculus, the latter shows the greatest variety both as to form and 
 size in the different types. Thus, for instance, it is usually very 
 small in Birds, while in Lizards (e,g., Lacerta) it is of considerable size 
 
 A 
 
 .c/r 
 
 FIG. 182. RIGHT MEMBRANOUS LABYRINTH OF A, Lacerta viridis, AND B, 
 Alligator mississipiensis, from the outer side. (After G. Retzius.) 
 
 ade, aperture of the ductus endolynrphaticus ; esc, sacculo-cochlear canal ; frt, 
 foramen recessus seal tympani ; tv, tegmentum vasculosum ; -sr, septum 
 cruciatum ; mb, membrana basilaris. (Other letters as in Figs. 180 and 181.) 
 
 The aperture of communication between the utriculus and sac- 
 culus persists, though it gradually becomes narrowed, as does also 
 that between the sacculus and cochlea. The connection between 
 these latter may be drawn out to form a canal (canalis reuniens), and 
 this is particularly the case in Birds ; in Crocodiles an inter- 
 mediate condition between Birds and Lizards is seen. The mem- 
 branous labyrinth of Birds, however, is always specially characterised 
 by the peculiar arrangement of the anterior and posterior 
 canals, which are greatly arched, and the position of their openings 
 into the sinus superior (canal commissure) is reversed. 
 
 In lower types (Swimming Birds) this peculiarity is less marked than in 
 the higher forms, and it would be interesting to ascertain the condition 
 of these parts in the Struthionidse, as one would expect to find in them 
 important points of connection with Reptiles. 
 
AUDITORY ORGAN 
 
 229 
 
 raa 
 mu.1 
 
 In many Reptiles the free end of the ductus endolymphaticus is 
 situated close under the roof of the skull beneath the parieto- 
 occipital suture, and in the Ascalabota the duct even leaves the 
 cranial capsule, passes back between the muscles of the neck, and 
 in the region of the pectoral arch becomes swollen to form a large 
 folded sac, from which finger-shaped processes extend to the 
 ventral surface of the vertebral column and to the sub-mucous 
 tissue of the pharynx. These processes may also branch out in a 
 labyrinthic manner into the 
 orbit, and they are always 
 filled with a white semi- 
 solid mass of calcareous 
 substance, as in Anura : 
 calcareous matter is present 
 in the ductus endolym- 
 phaticus of all Vertebrates, 
 at any rate in the embryo. 
 In Birds, the duct does not 
 pass out of the cranial 
 cavity. 
 
 A tympanic membrane 
 
 is present in Birds and all 
 
 D i'i , TT 4. FIG. 183. RIGHT MEMBRANOUS LABYRINTH 
 
 Reptiles except Hatteria, OF Turd , m mmicilS} from the inner side> 
 
 Snakes, and Amphisbse- (After G. Retzius. ) Letters as before, 
 nians ; and in the two last- 
 mentioned groups the tympanic cavity and Eustachian tube are also 
 wanting. In Crocodiles the tympanic cavity is very complicated, 
 and in them as well as in Birds, the two Eustachian canals open by 
 a single median aperture into the pharynx. The osseo-cartilaginous 
 coluinella is well developed in the Sauropsida, and is not distinct 
 from the stapedial plate ; in Hatteria it is continuous distally with 
 the hyoid (p. 92). 
 
 In certain Lizards (e.g., Ascalabota, Monitor), an indication of 
 the development of an external auditory passage is seen, the 
 tympanic membrane being partially covered posteriorly by a small 
 fold of skin, usually enclosing the anterior border of the digastric 
 muscle ; and in Crocodiles there is a definite integumentary valve 
 moved by muscles. In certain Birds also (e.g.. Owls), there is 
 a moveable valve. 
 
 Mammals. The auditory organ of Mammals reaches a much 
 higher stage of development (Fig. 184), but in Monotremes it 
 shows many points of resemblance to that of Reptiles and Birds. 
 
 The cochlea now reaches its highest development, and grows 
 into a long tube which becomes spirally coiled on itself. 1 In this 
 
 1 In Man it forms nearly three coils, and in other Mammals from one and a 
 half (Cetacea) up to as many as four or more. Thus in the Rabbit there are two 
 and a half, in the Ox three and a half, in the Pig almost four, and in the Cat 
 three coils. The cochlea, as well as the sacculus and all parts of the pars superior 
 of the membranous labyrinth, vary considerably both in form and arrangement in 
 the various types. 
 
230 
 
 COMPARATIVE ANATOMY 
 
 respect, as well as in the more highly-specialised histological struc- 
 ture of the cochlea, lies the characteristic peculiarity of the auditory 
 organ of Mammals. The auditory nerve forms the axis of the spiral. 
 In consequence of this development of the cochlea, the papilla 
 
 re 
 
 FIG. 184. RIGHT MEMBRANOUS LABYRINTH OF RABBIT (Lepus cuniculm.) 
 A, from the inner, and B from the outer side. (After G. Retzius.) 
 
 8us, sinus utrieularis sacculi ; esc, canalis reuniens Henseni ; rb, basilar branch of 
 the auditory nerve (ac) ; f, facial nerve ; mb, basilar membrane ; fis, spiral 
 ligament, (Other letters as Figs. 180-183.) 
 
 acustica, or, as it is called in Mammals, the organ of Corti, is drawn 
 out to a considerable length, and the part of the wall of the cochlea 
 on which it lies is called the basilar membrane, while the opposite 
 wall is spoken of as the membrane of Reissner (Fig. 186) : this is- 
 already represented in Crocodiles and Birds. 
 
AUDITORY ORGAN 231 
 
 The aperture of communication between the pars superior and 
 pars inferior of the membranous labyrinth that is, between the 
 sacculus and utriculus, is entirely obliterated in Mammals, the two 
 parts being only indirectly connected with one another by means 
 of the ductus endolymphaticus. This bifurcates at its point of in- 
 sertion into the membranous labyrinth, one limb being connected 
 with the utriculus and the other with the sacculus (Fig. 179) ; while 
 its upper end perforates the inner wall of the cartilaginous or bony 
 auditory capsule, passes into the cranial cavity, and terminates by 
 an expanded extremity (saccus endolymphaticus) in the dura mater. 
 Osmosis can thus occur between the lymph contained in the en- 
 dolymphatic and epicerebral lymph-spaces respectively. 
 
 The tympanic membrane is secondarily situated deep down in the 
 external auditory meatus, and thus an important difference is seen 
 between the Amphibia and Sauropsida on the one hand, and the 
 Mammalia on the other. The tympanic cavity and Eustachian tube 
 are well developed, and in place of the single bony columella of the 
 Sauropsida there is a chain of three auditory ossicles, articulating with 
 one another and extending between the tympanic membrane and 
 the fenestra ovalis. These are the malleus, the incus with its 
 orbicular apophysis, and the stapes}- 
 
 The stapedius muscle arises from the wall of the tympanic cavity, and i.s 
 inserted into the stapes, serving to keep the membrane of the fenestra 
 ovalis stretched. It is supplied by the facial nerve and corresponds to a 
 specialised portion of the hinder belly of the biventer, and can be traced back 
 as far as Fishes. A tensor tympani supplied by the mandibular division of the 
 trigeminal and derived from the internal pterygoid muscle (primarily from 
 the masticatory muscles of Fishes) also arises from the wall of the tympanic 
 cavity, and is inserted into the manubrium of the malleus, serving to keep 
 the tympanic membrane stretched. Both these muscles are composed of 
 striated fibres. 
 
 As already mentioned, the form of the membranous labyrinth 
 is repeated by its enclosing cartilaginous or bony capsule, which 
 forms, so to speak, a sort of cast around its individual parts. Thus 
 it is usual to speak of a cartilaginous or bony labyrinth as distin- 
 guished from the membranous labyrinth enclosed within it, the 
 two being separated by the perilymphatic cavity. In Mammals 
 the skeletal labyrinth becomes ossified before any other part of the 
 skull, and is incompletely divided into two parts enclosing the 
 utriculus and sacculus respectively. With the latter part is 
 connected the bony cochlea, the axis of which lessens in size from 
 base to apex (Fig. 185), and round it a bony lamella (lamina 
 spiralis ossea) winds in a spiral manner ; this extends into the 
 cavity of the coils of the cochlea without quite reaching the 
 opposite wall (Figs. 185 and 186), being continued outwards by 
 two laterally-diverging lamellae, mentioned above as the basilar 
 
 1 Cp. p. 100 and Figs. 80 and 233, in which the mode of development of these 
 parts is shown. There is often also a bony (interhyal) rudiment in the tendon 
 of the stapedius muscle. 
 
i>3i> COMPARATIVE ANATOMY 
 
 membrane and membrane of Reissner ; tbese lie at an angle to one 
 another and correspond to the inner walls of the membranous 
 cochlea or scala media, which is approximately triangular in 
 transverse section. The outer wall abuts against a portion of the 
 peripheral part of the bony cochlea (the region between Ls and 
 the peripheral end of R in Fig. 186). It is apparent therefore that 
 
 FIG. 185. BONY 'COCHLEA OF MAX. (After A. Ecker.) 
 
 A, axis ; Lso, Lso 1 , lamina spiralis ossea, the free edge of which, perforated by the 
 fibres of the auditory nerve, is visible at f ; H, hamulus. 
 
 FIG. 186. DIAGRAMMATIC TRANSVERSE SECTION OF THE COCHLEA OF A 
 
 MAMMAL. 
 
 KS, bony cochlea ; Lo, Lo l , the two layers of the lamina spiralis ossea, between 
 which at JVthe auditory nerve (together with the ganglion, left of L) is seen ; L, 
 limbus laminae spiralis ; B, membrana basilaris, on which the neuro-epithe- 
 lium lies ; R, Reissner's membrane ; Sv, scala vestibula ; St, scali tympani ; 
 8m, scala media (membranous cochlea) ; C, membrane of Corti ; Ls, liga- 
 mentum spirale. 
 
 the scala media does not by any means fill up the lumen of the bony 
 cochlea, but that a cavity is left on either side of it, corresponding 
 to those we have already met with in the auditory organ of Birds 
 and known as the scala vestibuli and scala tympani (Figs. 179 and 
 186). 
 
 Both of these are continuous with the perilymphatic cavity, 
 and, following the direction of the scala media, open into one 
 another at the blind end of the latter (Fig. 179). The scala 
 vestibuli is shut off from the tympanic cavity by the membrane 
 of the fenestra ovalis, to which the stapes is applied externally ; 
 the scala tympani is closed by the membrane of the fenestra 
 rotunda. 
 
 On the floor of the bony cochlea, not far from the fenestra 
 rotunda, is an opening leading into a narrow canal, the ductus 
 perilymphatictySt which serves as a communication between the 
 perilymphatic cavity and the peripheral lymphatic trunks of the 
 head (Fig. 179). 1 
 
 The fibres of the auditory nerve running along the axis of the bony 
 cochlea extend in their course laterally outwards, between the two plates 
 
 1 A ductus perilymphaticus can be plainly made out from Reptiles onwards. 
 
AUDITORY ORGAN 
 
 233 
 
 of the lamina spiralis ossea (Figs. 186, J87). On the free border of the 
 latter they pass out, and break up into terminal nbrilla? on the inner surface 
 of the basilar membrane. 
 
 The fibrillse extend to the sensory or auditory cells, and these are stretched 
 as in a frame between the firm supporting and isolating cells or bacilli. 
 From the surface of the bacilli a resistant net-like membrane (membrana 
 reticularis) extends laterally, and through the meshes of the latter the hairs 
 of the auditory cells project. The number of the outer hair-cells may be 
 estimated at about 12,000. The auditory cells are covered by a thick and 
 
 FIG. 187. THE ORGAN OF COKTI. (After Lavdowsky.) 
 
 Lo, Lo\ the two plates of the lamina spiralis ossea ; ^\ T , auditory nerve with 
 ganglion ; N 1 , N 2 , the nerve branching up into fibrillae and passing to the 
 auditory cells (G, G) ; Ba, Ba, bacilli, or supporting cells; Mz, membrana 
 reticularis ; C, membrane of Corti ; Ls, ligamentum spirale, passing into the 
 basilar membrane ; Sm, scala media ; K, membrane of Reissner ; B, B, basilar 
 membrane. 
 
 firm membrane the membrana tectoria s. Corti which perhaps acts as a 
 damper, and which arises from the labium vestibulare of the lamina spiralis 
 ossea. The whole extent of the basilar membrane consists of clear thread- 
 like and very elastic fibres, of which about 16,000 to 20,000 can be made out 
 in Man. 
 
 A true pinna or auricula (Fig. 188), attached to the border of 
 the external auditory meatus and projecting freely from the head, 
 occurs in Mammals only (comp. p. 229). It is supported by 
 cartilage, and the intrinsic and extrinsic muscles in connection 
 with it are supplied by the facial nerve. 
 
 The pinna arises from a series of rounded eminences 011 the first 
 and second visceral arches, around the hyoid (spiracular) cleft, the lower 
 part of which closes up, while the upper part gives rise to the external 
 
234 
 
 COMPARATIVE ANATOMY 
 
 auditory ineatus. These auricular eminences unite to form a nearly con- 
 tinuous ring, on which are later formed the characteristic protuberances 
 known as the helix, antihelix, tragus, and antitragus. The variations in the 
 form of the pinna which are seen in various Mammals concern essentially the 
 later formed portion which projects upwards and backwards from the head 
 (Fig. 188). 
 
 FIG. 88. THE PIXNA OF VARIOUS PRIMATES. 
 
 In A, the shaded portion (b) represents the zone of the auditory eminences of the- 
 embryo, the unshaded that of the later formed auditory fold. B, Man, 
 Baboon and Ox, drawn to the same scale and superposed : s', s," s, spina or 
 tip of the ear. C, Macacus rhesus, with upwardly directed tip ; and D, Cerco- 
 pithecus, with backwardly directed tip. E, Man : the muscles are indicated 
 as follows m.a, attolens auriculae ; m.a', antitragicus ; m.t, tragicus ;, 
 m.f, inconstant muscle, extending from the tragicus to the margin of the 
 helix ; m.h', helicis major ; m. h" helicis minor ; s, tip of the ear rolled over. 
 
 A -D, after Schwalbe ; E after Henle. 
 
F. ORGANS OF NUTRITION. 
 
 ALIMENTARY CANAL AND ITS APPENDAGES. 
 
 The alimentary canal consists of a tube which begins at the 
 aperture of the mouth, passes through the body cavity (ccelome), 
 and ends at the vent or anus}- Its walls consist of several layers 
 (Fig 214,), of which the mucous membrane, lining the cavity of 
 the tube, and the muscular layer external to this, extend throughout 
 the canal. The mucous membrane consists of a superficial 
 epithelium and a deeper connective-tissue layer, the outer part 
 of which, or sub-mucosa, forms a loose network extending to the 
 muscular layer. The epithelium is derived from the hypoblast, 
 with the exception of that lining the mouth and anus (stomodwum 
 and proctodceum*) which is epiblastic in origin (p. 5). The con- 
 nective tissue and muscular layers arise from the splanchnic layer 
 of mesoblast of the embryo ; and the muscular coat, consisting 
 almost entirely of unstriated fibres, supplied with nerves from the 
 sympathetic system, is, as a rule, divided into two layers, the inner 
 being constituted by circular, and the outer by longitudinal fibres. 
 These serve for the contraction or peristalsis of the wall of the gut, 
 and thus fulfil the double function of bringing the nutritive con- 
 tents of the latter into the closest possible relation with the whole 
 epithelial surface, and at the same time of removing from the body 
 the substances which have not been absorbed. Striated (voluntary) 
 muscular fibres, supplied by cerebral or spinal nerves, occur only at 
 the anterior and posterior ends of the canal. 
 
 An outer accessory serous coat, the peritoneum, encloses the 
 gut externally in the region of the ccelome. This is covered on its 
 
 1 In embryos of many Vertebrates (e.g., Elasmobranchii, Amphibia), a pig- 
 mented ridge of cells is formed on the dorsal side of the gilt in the head and trunk, 
 and gives rise to a rod lying close beneath the notochord. In certain cases it 
 remains for a time in connection with the gut by a series of segmental canals 
 which later disappear. The meaning and subsequent fate of this aub-notochordod 
 rod or hypochorda are not known. 
 
 2 Phylogenetically the proctodaeum is older than the stomodaeum, and in many 
 Vertebrates it is derived directly from the blastopore. 
 
236 COMPARATIVE ANATOMY 
 
 free surface by pavement epithelium, and, dorsally to the alimentary 
 canal, is reflected round the entire body-cavity, converting the 
 latter into a large lymph-sinus. A parietal layer, lining the body- 
 cavity, and a visceral, layer, reflected over the viscera, can thus be 
 distinguished in the peritoneum (Fig. 7). The part where one 
 passes into the other, which is thus primitively double, is called 
 the mesentery?- and this serves not only to support the alimentary 
 canal from the dorsal body-wall, but also to conduct the vessels 
 and nerves passing from the region of the vertebral column to the 
 viscera. With the lengthening of the alimentary canal during 
 development, the mesentery may give rise to a more or less com- 
 plicated system of folds in which the viscera are enveloped. 
 
 The most anterior section of the primitive alimentary tract 
 of the Ichthyopsida functions as a respiratory cavity as well as a 
 food-passage. A row of sac-like outgrowths, lying one behind the 
 other, are developed from the mucous membrane and eventually 
 unite with the ectoderm, apertures being formed to the exterior 
 (Fig. 189, A). Between the channels thus formed, the visceral 
 arches (p. 69) are situated, and along the latter certain vessels 
 are formed by means of which a continual interchange of gases can 
 take place between the blood and the air contained in the water 
 passing through the sacs. In this manner the gills or branckice 
 (p. 273) arise. Even in the Amniota, although gills are not 
 developed, the larger portion of the cavities of the mouth and 
 pharynx lying behind the internal nostrils serves as a common air- 
 and food-passage until a proper palate (pp. 92, 202) is formed (Fig. 
 189, C). 
 
 With the formation of a definite palate (most Amniota), the 
 primitive mouth-cavity becomes divided into an upper respiratory, 
 and a lower nutritive portion that is, into a nasal and a secondary 
 or definitive mouth-cavity. The separation, however, is not a com- 
 plete one, the passage being common to both cavities for a certain 
 region (Fig. 189, D). This region, in all Vertebrates, is called 
 the pharynx, and in Mammals it is partially separated from the 
 mouth by a fibrous and muscular fold, the velum palati, or free 
 edge of the soft palate? 
 
 The alimentary canal of Vertebrates is typically divisible into 
 the following principal sections (Fig. 190) : Mouth or oral 
 cavity, pharynx, gullet or cesophagus, stomach, small intestine, and 
 large intestine. The large intestine may communicate with a 
 cloaca, into which the urinary and genital ducts also open, or it 
 may open directly to the exterior. The small intestine may be 
 further differentiated into duodenum, jejeunum and ileum, and the 
 large intestine into colon and rectum. A blind-gut or cwcum is 
 
 1 In Mursenoids, Dipnoans, and Lepidosteus, a ventral mesentery is also present, 
 but in Lepidosteus it only extends for a short distance along the hinder part of 
 the gut. 
 
 2 A membranous velum palati exists in Crocodiles. 
 
ALIMENTARY CANAL AND ITS APPENDAGES 237 
 
 o 
 
 Fio. 189, DIAGRAMS OF THE ORAL CAVITY AND PHARYNX OF A FISH (A), 
 AMPHIBIAN (B), REPTILE OR BIRD (C), AND MAN (D). 
 
 N t external nostril ; Ch, internal nostril ; D, alimentary canal ; K, gill-slits ; 
 L, lung; T, trachea; O, oesophagus: the arrow marked A indicates the 
 respiratory passage, that marked R the nutritive passage ; f, the point where 
 the two passages cross one another. 
 
238 
 
 COMPARATIVE ANATOMY 
 
 often present at the junction of the large and small intestine. 
 Between the stomach and duodenum as well as between the ileum 
 
 FIG. 190. DIAGRAM OF THE ALIMENTARY CANAL OF MAN. 
 
 salivary glands ; Ph, pharynx ; Gl.th, thyroid ; Cll.thy, thymus ; Ly, lung ; 
 Oe, oesophagus ; Z, diaphragm ; My, stomach ; Lb, liver ; Pa, pancreas ; 
 J)d, small intestine ; Vic, ileo-colic valve ; Pv, vermiform appendix (caecum) ; 
 Ca, Ct, Cd, ascending, transverse, and descending portions of the colon ; A', 
 rectum ; A , anus. 
 
 and large intestine there is, as a rule, a marked elevation of the 
 muscular coat serving as a sphincter (pyloric and ileo-colic valves). 
 There is also a sphincter muscle at the anus. 
 
TEETH 239 
 
 The small intestine is in most cases the longest section of the 
 alimentary tract : the bile and pancreatic ducts open into its 
 anterior portion. 
 
 In almost all cases the alimentary canal becomes more or less 
 coiled, and thus presents a greater surface for absorption. As a 
 general rule, it is relatively longer in herbivorous than in carni- 
 vorous animals. A considerable increase of surface also commonly 
 results from the elevation of the mucous membrane to form folds, 
 villi, and papillae (p. 269). 
 
 Certain appendages are present in connection with the ali- 
 mentary canal. These are all developed primarily from the 
 hypoblast and are thus of epithelial origin : mesoblastic elements 
 are added to them secondarily. Whether they function as 
 glands throughout life or not, they are all formed on the same 
 type as glands. 
 
 Beginning from the mouth the following appendicular organs 
 may be distinguished (Fig. 190) : 
 
 (1) Mucous and salivary glands. 
 
 (2) The thyroid. 
 
 (3) The thymus. 
 
 (4) The lungs or air-Madder. 
 
 (5) The liver.. 
 
 (6) The pancreas. 
 
 In addition to these, gastric and intestinal glands are embedded 
 in the wall of the gut. 
 
 MOUTH. 
 
 In Amphioxus the entrance to the mouth (oral hood) is pro- 
 vided with cirrhi, and in Petromyzon 1 it is surrounded by a ring of 
 cartilage (Fig. 54) : all other Vertebrates are provided with jaws. 
 
 Definite lips provided with muscles first appear in Mammals, 
 but are wanting in Monotremes. The space between them and the 
 jaws is spoken of as the vestibulum oi*is ; this may become 
 extended on either side to form cheek-pouches, which serve as food 
 reservoirs (many Monkeys and Rodents). 
 
 The chief organs of the oral cavity are the teeth, the glands, 
 and the tongue. 
 
 Teeth. 
 
 The teeth are developed quite independently of the endo- 
 skeleton, and both epiblast and mesoblast take part in their forma- 
 tion (comp. p. 30). The first traces of the teeth are seen primarily 
 
 1 The mouth of the Lajnprey serves as a suctorial organ for attaching the 
 animal to foreign objects. The larva? of Lepidosteus and Anura are temporarily 
 provided with suctorial organs 
 
240 
 
 COMPARATIVE ANATOMY 
 
 in the form of superficial papillae of the mucous membrane ; but 
 secondarily, owing to want of space, the epithelium of the mouth 
 grows inwards so as to give rise to a dental lamina which becomes 
 enlarged distal ly at certain points to form the so-called enamel-organs. 
 These as they grow deeper into the rnesoblast become bell-shaped, and 
 enclose modified masses of connective-tissue, the dental papilla'' ; 
 the upper cells of the papillaB, i.e., those next to the enamel-organ 
 are known as odontoUasts (Fig. 191, A). The epithelial and con- 
 nective tissue germs come into the closest relation with one another 
 
 FIG. 191 A. DIAGRAM OF THE DEVELOPMENT OF A TOOTH. 
 
 EM, epithelium of mouth ; SK, dental lamina ; ZK, dental papilla ; Ma, mem- 
 brana adamantina of enamel-organ ; 0, odontoblasts ; DS, dentine; By, Bg, 
 connective tissue follicle or sac surrounding the tooth. 
 
 FIG. 191i5. SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH 
 
 A TOOTH. 
 
 ZS, enamel ; ZB, dentine ; ZC, cement ; PA 1 , aperture of the pulp-cavity (PH). 
 
 and give rise respectively to the calcified enamel and dentine (ivory'), 
 of which the teeth are composed. The enamel is the harder and 
 contains little organic matter, and the dentine is permeated by a 
 system of fine canals in which are delicate processes of the odonto- 
 blasts. A third, bone-like substance, the cement, is also formed 
 round the bases of the teeth, and between the folds of enamel 
 when these are present ; it may unite with the bones of the jaw. 
 The root of the tooth, embedded in the gums, is provided at its 
 
TEETH 
 
 241 
 
 lower end with an opening leading into the central pulp-cavity 
 (Fig. 191, B), into which blood-vessels and nerves extend. 
 
 In most Vertebrates below Mammals all the teeth are essentially 
 similar in form (liomodont dentition} : in Mammals, on the other 
 hand, they become differentiated into distinct groups (heterodont 
 dentition}, known as incisors, canines, and cheek-teeth or grinders 
 (premolars and molars}. 
 
 A succession of teeth takes place throughout life in almost all 
 Vertebrates except Mammals, in which there are very exceptionally 
 more than two functional sets, the so-called milk- or deciduous teeth 
 and the successional teeth. This difference is expressed by the 
 terms polyphyodont and diphyodont. (Comp. p. 245). 
 
 Fishes, Dipnoans, and Amphibians. The homology and 
 similarity of the teeth with the dermal denticles of Elasmobranchs 
 has already been treated of (p. 30). The most primitive form of the 
 tooth is that of a simple cone, but even amongst Elasmobranchs, 
 in which the teeth are arranged in 
 numerous parallel rows upon the car- 
 tilaginous jaws, this form has already 
 become modified in various ways for 
 seizing or crushing the food. 
 
 Of those Anamnia which possess 
 a bony skull, four groups of tooth- 
 bearing bones may in general be dis- 
 tinguished, viz., (1) the maxillary arch 
 (premaxiUa and maxilla] ; (2) the 
 palatal arch (vomer, palatine, ptery- 
 goid) ; (3) the (unpaired) parasphe- 
 noid : and (4) the rnandibular arch 
 (dentary and splenial). 1 
 
 True teeth are wanting in Cyclo- 
 stomes, and amongst cartilaginous 
 Ganoids they are absent in the 
 adult Sturgeon, though rudiments 
 are present in the embryo. Amongst 
 Teleostei they are wanting in the 
 adult Lophobranchii and in Coregonus. In the Cyclostomes they 
 are represented functionally by a number of conical horny teeth. 2 
 
 In bony Ganoids and Teleosts, teeth may be present on all the 
 bones bounding the oral cavity, as well as on the hyoid and the 
 branchial arches (" pharyngeal bones"). In the latter position, 
 as well as on the parasphenoid, they often form brush-like 
 groups. In form the teeth may be cylindrical, conical, or 
 hooked ; or they may be chisel-shaped (Scarus, Sargina9) , 
 
 1 The teeth of Elasmobranchs may be compared to (2) and (4) of these. 
 
 2 Structures bearing a superficial resemblance to vestigial true teeth are 
 recognisable beneath the horny teeth, but they possess no odontoblasts or 
 enamel epithelium. 
 
 FIG. 192. SKULL OF Batrachowps 
 attenuatus. (From the ventral 
 side, showing the teeth on the 
 parasphenoid. ) 
 
242 
 
 ZK 
 
 COMPARATIVE ANATOMY 
 
 / .3 2 ' 3, ^ 
 
 - -RP 
 
 ZK 
 
 JIF 
 
 c.c 
 
 B ^O c 
 
 FIG. 193, A. TOOTH OF FROG, AND 193, B, Salamandra atra. 
 
 ZK, crown ; ZS, base ; RF, circular furrow ; S, apex, covered with enamel ; 
 PH, pulp-cavity ; M, maxilla. 
 
 FIG. 193, C. TRANSVERSE SECTION THROUGH PORTION OF A TOOTH OF A LABY- 
 RINTHODONT (Mastodonsaurux). (After J. Storrie. ) 
 
 C.c, central pulp-cavity ; 1, inflections of the enamel, surrounded by dentine, 
 which reach the centre (c.c) ; 2, half-length inflections between 1 ; 3, short 
 inflections between 1 and 2. 
 
 resembling the incisors of Mammals, and working together like 
 scissors; in some Fishes they give rise to a definite pavement, 
 are rounded in form, and serve to crush the food. They may, 
 
TEETH 243 
 
 agjain, be delicate and bristle-like (Chsetodon), or sabre-shaped 
 (Chauliodus). 
 
 In the Dipnoi (Fig. 62) the teeth are compound and exceedingly 
 massive, presenting sharp edges and points. 
 
 In the Amphibia there is in general a considerable diminution 
 in the number of teeth as compared with Fishes ; and at the same 
 time a much more uniform character is noticeable in their form 
 throughout (Fig. 193, A, B). They are enlarged conically below, and 
 rest on a definite base, while above they become narrower and 
 slightly curved, ending either in a double (Myctodera, Anura), or 
 a single apex (Perennibranchiata, Derotremata, Gymnophiona); the 
 latter is the more primitive condition. The teeth lie deeply em- 
 bedded in the mucous membrane, and are present, as a rule, on the 
 preinaxiila, maxilla, and mandible (except in Anura), as well as on 
 the vomer and palatine, but rarely on the parasphenoid (certain 
 TJrodeles, Fig. 192) ; in the larvae of Salamanders and in Proteus the 
 splenial of the lower jaw is also toothed. Horny teeth and horny 
 jciu's, developed entirely from the epidermis, are present in larval 
 Anura, and similar structures occur in Siren lacertina. 
 
 Teeth are altogether absent in the Bufonidse and in Pipa. 
 
 The teeth of certain of the Stegocephala (Labyrinthodonta) 
 were extremely complicated, the enamel appearing as numerous 
 corrugated folds extending from the periphery towards the centre 
 (Fig. 193, C). 
 
 Reptiles and Birds. Corresponding with the greater firm- 
 ness of the skull in Reptiles, the dentition is usually strongly 
 developed, and occasionally at the same time it is more highly 
 differentiated than in Amphibians. The teeth are either situated 
 upon a ledge on the inner side of the lower jaw, with which they 
 become fused basally (pleurodont dentition most Lacertilia) ; or 
 they lie on the free upper border of the jaw (acrodont dentition 
 Chameleon) ; or finally ,as in Crocodiles and numerous fossil Reptiles, 
 they are lodged in alveoli (thecodont dentition) (Fig. 194, A, a, b, c). 
 Both upper and lower jaws, and occasionally the palatine and 
 pterygoid also, are toothed (Lizards and Snakes) ; and in Hatteria, 
 vornerine teeth may also be present. The teeth are usually conical 
 and more or less pointed, but in Lizards the apex is double, and in 
 many Reptiles (e.g., Palaeohatteria, Hatteria, Uromastix spinipes, 
 Agama?, and numerous fossil forms, especially the Theriodontia of 
 the Trias of South Africa), a heterodont dentition is already indi- 
 cated. Almost all Reptiles are polyphyodont. 
 
 In poisonous Snakes a varying number of maxillary teeth are 
 differentiated to form poison-fangs. Thus in the common Viper 
 (Pelias berus) there are on each side ten poison- fangs arranged 
 in transverse rows ; the stronger ones project freely, while the lesser, 
 reserve teeth lie within the gum (Fig. 195, A) ; only one of these 
 teeth, however, is firmlv fixed to the maxilla at a time. Each fang 
 
 K 2 
 
244 
 
 COMPARATIVE ANATOMY 
 
 is perforated by a poison-canal, which is incompletely surrounded 
 by the pulp-cavity, the latter having the form of a half-ring in 
 
 FIG. 194. A, DIAGRAMS OF TRANSVERSE SECTION THROUGH THE JAWS OF 
 REPTILES, SHOWING PLEURODONT (a), ACRODONT (6), AND THECODONT (c) 
 DENTITIONS. B, a, LOWER JAW OF Zootoca vivipara; b, OF Anguis fragilis. 
 (After Leydig. ) 
 
 transverse section (Fig. 195, B, C,) : the duct of the poison-gland 
 passes into an aperture at the base of the tooth which leads into 
 
 FIG. 195. FIGURES OF THE POISON-FANGS OF A VIPERINE SNAKE. 
 
 A t skull of Rattlesnake; B, transverse section through about the middle of the 
 poison-fang of Vipera ammodytes ; C, transverse section through the poison- 
 fang of Vipera ammodytes near its distal end. (B and C after Leydig.) 
 
 Gz, poison-fang ; /?z, reserve fangs ; GO, poison-canal ; PH, pulp-cavity. 
 
 the poison-canal, and the latter opens at a short distance from the 
 apex of the tooth (see the course of the arrow in Fig. 195, A). 
 
TEETH 245 
 
 Between the ordinary teeth of Snakes and the poison-fangs with closed 
 canals, there are numerous intermediate forms in which certain of the teeth 
 are simply grooved along their anterior side. A similar condition is also 
 seen in the teeth of the lower jaw of a poisonous Mexican Lizard (Heloderma). 
 (Comp. p. 252.) 
 
 A peculiar tooth is present in the embryos of Lizards and some Snakes. 
 It projects considerably beyond its neighbours, and lies in the median line of 
 the lower jaAv, extending vertically towards the snout and serving the young 
 as a means of breaking through the parchment-like egg-shell. This must 
 not be confounded with the horny "neb" in Crocodiles, Chelonians, Birds, 
 and Monotremes amongst Mammals, which is of a purely epithelial nature. 
 
 Chelonians, like existing Birds, are provided with horny sheaths 
 to the jaws instead of teeth. The presence of teeth in the embryo 
 of Trionyx, as well as of a rudimentary dental lamina in embryos of 
 Chelone and Sterna, for example, proves, however, that this is only 
 a secondary condition. 
 
 In the cretaceous Birds of N. America (Odontornithes) teeth were 
 present, and were either situated in a definite alveoli (Ichthyornis), 
 or simply in grooves (Hesperornis). The premaxillse were tooth- 
 less, and seem to have possessed a horny beak. The single-pointed, 
 smooth teeth of Archaeopteryx were probably situated in alveoli. 
 It is possible that some of the Eocene Birds (e.g.> Argillornis, 
 Gastornis) possessed teeth. 
 
 Mammals. The heterodonb dentition characteristic of the 
 Mammalia as a Class must have arisen by a modification of a 
 simple homodont condition, in which the teeth were all conical 
 and of similar size and shape. Side by side with this modifi- 
 cation, a shortening of the jaws has usually taken place, and 
 the teeth serve not only to seize and bite the food, but also to 
 masticate it and to test its qualities. The frequent presence of 
 rudimentary, functionless teeth, renders it probable that in the 
 course of phylogenetic development the teeth have undergone a 
 decrease in number. 1 An increase in number, such as is met with 
 in toothed Whales, is due to the separation, during ontogeny, of 
 the component cusps of complex teeth, and is therefore not a 
 primitive, but a highly specialised condition. 
 
 As already mentioned, the succession is nearly always reduced 
 to two functional sets, the so-called milk or deciduous teeth and the 
 successional or permanent teeth, and in some cases (see p. 249) even 
 one of these may be rudimentary. Traces, however, of an earlier 
 set occur in certain Mammals : this may be spoken of as a " pre- 
 milk dentition." Occasionally also (e.g., in Man) one or more 
 teeth appear which replace the corresponding " permanent " teeth 
 and thus indications of four and possibly even of five sets can 
 in all be recognised. 
 
 In each of the two functional sets, incisors, canines, and cheek- 
 
 1 The last molar of Man, or so-called " wisdom-tooth," seems to be gradually 
 disappearing ; it appears last and is lost first, and often does not reach the 
 grinding surface. In many cases also the outer upper incisors are wanting. 
 
246 COMPARATIVE ANATOMY 
 
 teeth, or grinders, can. as a general rule be distinguished. The 
 teeth which replace the milk -grinders are distinguished a&premolars 
 from the molars, which are situated further back in the jaw and 
 have no predecessors. 1 
 
 All the teeth are imbedded in well-developed alveoli of the 
 jaw-bones, the upper incisors being situated in the premaxilloe, the 
 upper canines and cheek-teeth in the maxilla?, and the lower teeth 
 in the mandible (dentary). The canine, which corresponds to a 
 specially differentiated premolar, and is most characteristically 
 developed in Carniyora, lies in a- more or less continuous series 
 with the incisors. The premolars follow behind the canine, the 
 space usually present between them being called the diastema, and 
 then come the molars. The primary arrangement of the teeth is 
 such that there is an alternation between those of the upper and 
 lower jaw : thus the teeth in one jaw do not usually correspond in 
 position with those of the other, but with the interspaces between 
 them. 
 
 In some cases the enamel-organ persists in all the teeth, 
 which then continue to grow throughout life (e.g., Lepus) ; in. 
 others this is true of the incisors only (e.g., numerous Rodents, 
 Elephant) ; but more usually growth ceases after a certain time, 
 and the teeth then form de finite fangs or roots, each perforated by a 
 small canal communicating with the reduced pulp-cavity. 
 
 The incisors are usually chisel-shaped, while the canines, in 
 those cases where they are most characteristically developed 
 (Carnivora), possess a pointed, conical form, and are more or 
 less curved. The cheek-teeth either possess sharp, cutting crowns 
 (e.g., Carnivora), or the crowns are broad and more or less flat and 
 tuberculated, and adapted for grinding the food. In the latter 
 case the relations of the enamel, dentine, and cement are such as 
 to produce an uneven surface with wear, showing a characteristic 
 pattern in the different groups (Figs. 196200). 
 
 The relations and number of the tubercles which may be conical (e.g., Pig) 
 or cresceiitic (e.g., Horse, Ruminants, Fig. 199), as well as the form of the 
 teeth in general, is of great importance in elucidating the ancestral history 
 of the Mammalia, and attempts have been made to trace the evolution of 
 the various forms of molar met with in the Class. According to one view 
 the tuberculated molar has arisen by the gradual modification of a single 
 conical tooth, which has produced lateral outgrowths or buds. Thus taking the 
 simple conical form such as exists in toothed Whales as the most primitive 
 form of mammalian tooth, we find that certain extinct Mammals (e. g. , Trico- 
 nodon) possessed teeth with a main cone and two lateral cusps. It has been 
 supposed that the more complicated forms have been derived from this 
 triconodont tooth firstly by a rotation of the lateral cusps outwards in the 
 upper, and inwards in the lower tooth, thus forming a tritubercular tooth, 
 with three cusps arranged in a triangle ; and secondly by the addition of other 
 cusps, the first to appear being the posterior heel or talon. 
 
 Another hypothesis is that the mammalian cheek-teeth were primarily 
 
 1 It must, however, be remembered that in some cases the so-called pre- 
 molars have no predecessors (see p. 249). 
 
TEETH 
 
 247 
 
 prn 
 
 FIG. 196. DENTITION OF THE DOG (Canis familiaris). 
 
 FIG. 197. DENTITION OF THE HEDGEHOG (Erinaceus europwus). 
 (The teeth of both jaws from the side, and those of the upper jaw from below.) 
 
 i, incisors ; c, canines ; pm, premolars ; m, molars. 
 
 multitubercular, having originated by the fusion of a number of simple coni- 
 cal teeth ; and certain facts in their development and the presence of multi- 
 tuberculate Mammals in the Triassic rocks, as well as a comparison with 
 the massive teeth met with in various Fishes for example, seem to sup- 
 port this view. The resulting decrease in number is compensated for by the 
 greater perfection of the individual teeth. 
 
248 
 
 COMPARATIVE ANATOMY 
 
 FIG. 198. DENTITION OP THE PORCUPINE (Hystrix hirsutirostris). 
 
 FIG. 199. DENTITION OF SHEEP (Ovis aries). 
 
 (References as before, but in Fig. 199 'the teeth of the lower instead of the 
 upper jaw are figured from the surface. ) 
 
TEETH 
 
 249 
 
 The limitation of the succession to two or even one functional 
 set is probably due to the concentration of several successive 
 generations of teeth in correspondence with the higher develop- 
 ment of the individual tooth. This concentration is most marked 
 in Marsupials, in which only a single tooth, usually described as the 
 fourth premolar, has a predecessor. Differences of opinion exist as 
 to whether this tooth is to be regarded as the last remains of the 
 first or of the second set, or whether it belongs to the same series 
 as the others and is only retarded in development. The fact that 
 
 FIG. 200. DENTITION OF A CATARRHINE MONKEY (Nasalis larvatus). 
 References as before. 
 
 in toothed Whales the milk-teeth persist and the second set is 
 only represented in rudiment, seems to indicate that the teeth of 
 Marsupials, except the fourth " prernolar " belong to the first set, 
 and that the milk dentition of Mammals is not a secondary acqui- 
 sition. In other words, the primitive Mammalia were at least 
 diphyodont, and the apparent monophyodont condition seen (e.g., in 
 toothed Whales) is a secondary condition. In the placental Mammals 
 the second dentition becomes of greater importance than the first. 1 
 
 1 In many instances, however, the first premolar appears to belong to the 
 milk dentition, and this may possibly be the case as regards the molars also. 
 
250 COMPARATIVE ANATOMY 
 
 In describing the teeth of a Mammal it is convenient to make use of a 
 dental formula in which their number and arrangement can be seen at a 
 glance, the teeth of one side only being represented. Thus the adult dental 
 formula of those animals, the teeth of which are represented in Figs. 196 to 
 200, would be 
 
 Fig. 196. Dog, l'.\\\\l = 42 
 
 3*1*3*3 
 
 ,, 197. Hedgehog, 2 . i 2~3 = ^ 6 
 
 ,, .198. Porcupine, ^ . Q . ^ . 3 = 20 
 
 3 ' 3 
 199. Sheep, g . 1 . 3 ~ 3 =32 
 
 2*1 ' 2 * 3 
 ,, 200. Catarrhine Monkey, OVVTJTTQ ~ ^ 
 
 The most complete dentition is seen amongst Marsupials, the dental formula 
 of Myrmecobius being 3.1.3.5 g = 50 52. The more typical arrange - 
 
 3-1-4-3 
 
 ment is 3 . x . 4 . 3 - 44. 
 
 Sexual differences in dentition exist in a number of Mammals. Thus in 
 the male Wild Boar, Narwhal (Monodon), Dugong (Halicore), and Musk-deer 
 a modification of certain of the teeth (the canines or the incisors) to form 
 tusks occurs, and these serve as fighting weapons. In the Elephant and 
 Walrus tusks are present in both sexes : in the former they correspond to 
 incisors, and in the latter to canines. 
 
 In Ornithorhynchus the teetli become replaced functionally after 
 a time by the development of horny masticatory plates, 1 and in 
 Echidna they are wanting altogether. Adult Whalebone Whales and 
 certain Edentates (Myrmecophaga, Manis) are toothless, but rudi- 
 ments of teeth exist in the embryo. In other Edentates the teeth 
 are wanting in enamel. Canines are absent in certain Mammals 
 (e.g., Rodents) and the incisors may also be wanting. In the 
 typical Ruminants incisors and canines are present in the lower 
 jaw only. 
 
 Glands of the Mouth. 
 
 The glands of the mouth, like those of the orbit and integu- 
 ment, appear first in terrestrial Vertebrates, that is, from Amphi- 
 bians onwards. They have the function of keeping moist the mucous 
 membrane which comes into contact with the outer air. From 
 being at first almost entirely unspecialised, and giving rise simply 
 to a slimy fluid, they become differentiated later into structures the 
 secretions of which take on a very important function in relation to 
 digestion ; they may also, as in the case of poisonous Snakes and 
 Lizards, constitute dangerous weapons of offence. 
 
 With their gradually increasing physiological importance a 
 
 1 Horny crushing plates are also present in the Sirenia, the existing forms of 
 which possess numerous teeth, while the extinct Rhytina was toothless. 
 
GLANDS OF THE MOUTH 251 
 
 greater morphological complication both as regards number and 
 arrangement takes place. Their histological character also 
 becomes changed in such a manner that the most varied forms 
 of glands may be recognised. 
 
 Amphibians. With the exception of the Perennibranchiata 
 Derotremata, and Gymnophiona, a tubular gland becomes developed 
 in all Amphibia from the anterior portion of the roof of the mouth 
 (com p. Fig. 160), the main mass of which in Urodeles lies in the 
 cavity of the nasal septum or premaxilla (intermaxillary or inter- 
 nasal glancT). In Anura its position is more anterior than in 
 the Urodela, and it is more largely developed ; but in both cases 
 the ducts open on to the anterior part of the palate. 
 
 In Anura there is a second gland (pharyngeal gland) present in 
 the region of the internal nostrils, the secretion of which passes 
 partly into the latter and partly into the pharynx. 
 
 Numerous gland-tubes are also present in the tongue of 
 Amphibians, and in the Gymnophiona oral glands are abundant. 
 
 Me :- Km 
 
 Gc 
 
 FIG. 201. THE POISON- APPARATUS OF THE RATTLESNAKE. 
 
 S, the fibrous poison-sac, which is surrounded by the constrictor-muscle, Me at 
 J/c 1 an extension of the latter towards the lower jaw can be seen ; Gc, the 
 duct arising from the poison-gland, which passes into the poison-fang at f ; 
 the latter is embedded in a large sac of the mucous membrane, zf; Km, 
 masticatory muscles, some of which are seen cut through at * ; posterior to 
 this the cut edge of the scaly integument is seen ; oV, external nostril ; A, 
 eye displaced towards the antero^dorsal aspect ; z, tongue ; za, aperture of the 
 poison-fang. 
 
 Reptiles. The mouth-glands in Reptilia show an advance, on 
 those of Amphibia inasmuch as they are separated into groups. 
 Thus not only is there a palatine gland, homologous with the 
 intermaxillary gland, but lingual and sublingual, as well as upper 
 and lower labial glands are present. Chameleons and Snakes 
 are distinguished by a remarkable richness in glands, which 
 become most specialised into definite groups in the latter. In 
 poisonous Snakes the poison-gland becomes differentiated from 
 a portion of the upper labial gland. It is enclosed in a strong 
 
252 COMPARATIVE ANATOMY 
 
 fibrous sheath, and is acted upon by powerful muscles, so that its 
 secretion can be poured with great force into the duct (Fig. 201), 
 and thence into the poison-fang (p. 243). 
 
 The sublingual gland of a Mexican Lizard, Heloderma, has a similar 
 poisonous nature. The secretion passes out through four ducts, which 
 perforate the bones of the lower jaw in front of the grooved teeth (p. 245). 
 
 In marine Chelonians and Crocodiles there are no large glands united 
 into groups connected with the mouth. 
 
 Birds. In Birds, and more especially in climbing Birds 
 (Scansores), a well-developed lingual gland is present opening on 
 the floor of the mouth, as well as a gland at the angle of the 
 latter. There is no doubt that the lingual glands are homologous 
 with those of Lizards, but it is not known whether the gland at the 
 angle of the mouth corresponds with the posterior upper labial 
 gland of Reptiles that is to the poison-gland of Snakes. The 
 palatine glands of Birds are not homologous with those of Reptiles 
 and labial glands are wanting. 
 
 Mammals. Three sets of salivary glands may be distin- 
 guished in connection with the mouth in Mammals, which are 
 called, according to their position, (1) parotid, (2) submaxillary, 
 and (3) sublingual. Each of the two former of these opens into 
 the mouth by a well-defined duct, that of the sublingual having 
 several independent ducts. A special retrolingual portion usually 
 becomes differentiated from the sublingual gland and commu- 
 nicates with the submaxillary duct. 
 
 The parotid is usually situated at the base of the external 
 ear : its origin is not known. The submaxillary is a compound 
 gland, consisting of glandular elements which differ from one 
 another histologically : it lies beneath the mylohyoid muscle, close 
 to which the retrolingual gland is also situated; the latter is 
 wanting in only a few Mammals (e.g., Rabbit, Horse). The sub- 
 lingual gland extends between the tongue and the alveoli of the 
 teeth, and is rarely absent (e.g., Mouse, Mole). 
 
 With the exception of the parotid, all these glands, together 
 with certain smaller and less important ones, are homologous with 
 the oral glands of lower Vertebrates. 
 
 Salivary glands are wanting in the Cetacea. 
 
 Tongue. 
 
 Fishes. The tongue is, as a rule, rudimentary in Fishes, and 
 is simply represented by a fold of mucous membrane covering the 
 basi-hyoid, which in all the higher Vertebrates serves as a point of 
 origin for many of the lingual muscles. Except in Cyclostomes, 
 where it has to do with the suctorial apparatus, the tongue of 
 
TONGUE 
 
 253 
 
 Fishes is not capable of movement apart from the visceral 
 skeleton, and is wanting in a proper musculature. It is provided 
 with papillae and serves only as a tactile organ, or, when provided 
 with teeth (e.g., certain Teleostei, Fig. 60), as a prehensile organ 
 also. 
 
 In Dipnoans the tongue is not more highly differentiated 
 than in many Fishes. 
 
 Amphibians. In the Perennibranchiata (e.g., Proteus) there 
 is a little advance on the condition seen in Fishes, but in all other 
 Amphibia except the Aglossa (Pipa and Xenopus), in which it has 
 become degenerated, the tongue reaches a higher stage, owing to 
 the development of definite muscles which render an independent 
 movement of the organ possible, 
 as well as of glands. The tongue, 
 moreover, is relatively larger, 
 and the numerous papillae render 
 the surface velvet-like. Its 
 mobility varies greatly in the 
 different forms. It is usually 
 
 attached only by the anterior c 
 
 end or by a portion of its ventral ( ~~~ 
 surface (Fig. 202) : in other cases 
 
 '. frpp 1] rnnnrl anrl in FlG. 202. FIGURE SHOWING THE TONGUE 
 
 und, and in OF THE FRQG JX THREE DlFFERENT 
 Spelerpes (Fig. 203) is capable POSITIONS. 
 of being extended far out of the 
 
 mouth by means of a complicated mechanism, similar to that 
 which occurs in the Chameleon amongst Reptiles. 
 
 FIG. 203. -HEAD OF Spelerpes fuscm, WITH THE TONGUE EXTENDED. 
 
 Reptiles. In most Reptiles the tongue is usually freely 
 moveable, but its form and relative size varies greatly l (Fig. 
 204, A to E). It is provided with numerous sensory organs, 
 but no glands are present in the tongue itself. It is least mobile 
 in Chelonians and Crocodiles : in Snakes and many Lizards it is 
 forked at the apex, and in the Chameleon it is protrusible, as in 
 Spelerpes. 
 
 Birds. The tongue of Birds is usually poorly provided with 
 muscles. It possesses a horny covering, usually provided with 
 papillse and pointed, recurved processes; it may, as in many 
 
 1 Thus in Lizards the tongue is used for classificatory purposes ( Vermilinguia, 
 Crassihnyuia, Brevilinguia, Fissilinguia). 
 
254 
 
 COMPARATIVE ANATOMY 
 
 FIG. 204. A, TONGUE, HYOID APPARATUS, AND BRONCHI OF A GECKO (Phylo- 
 dactylus europaus) ; B, TONGUE OF Lacerta ; C, OF Monitor indicus ; D, OF 
 Emys europcm ; E, OF AN ALLIGATOR. 
 
 Z, tongue ; ZK, body of hyoid ; VH and HH, anterior and posterior eornua of 
 hyoid ; K, larynx ; Th, thyroid ; r J\ trachea ; B, bronchi ; Lg, lung ; J/, 
 mandible ; L, glottis ; ZS, sheath of tongue. 
 
THYROID 255 
 
 Reptiles, be split up at its distal end, being either bifurcated 
 (Trochilidse) or having a brush-like form. In Woodpeckers (the 
 extraordinarily developed epibranchials of which have already been 
 mentioned in the chapter on the skull), the tongue may be thrown 
 far out from the mouth by means of a complicated system of 
 muscles, and it thus serves as a prehensile organ. 
 
 The tongue is largest in predatory Birds (Rapaces) and Parrots, but its 
 size is here not due so much to the special development of muscles as to the 
 presence of fat, vessels, and glands. 
 
 Mammals. The tongue reaches its most complete morpho- 
 logical and physiological development in Mammals, and, as else- 
 where, undergoes the most various modifications in form. It is 
 as a rule flat, band-like, rounded anteriorly, and extensile. In- 
 trinsic as well as extrinsic muscles are well developed. A fold, 
 the so-called sublingua (plica fimbriata), is present on the lower 
 surface of the tongue, and is especially well marked in 
 Lemurs ; in the Slender Loris (Stenops) it is supported by carti- 
 lage. This probably corresponds to the last vestige of the tongue 
 of lower Vertebrates which has been replaced by the more highly- 
 developed organ characteristic of Mammals. The latter has pro- 
 bably arisen from the posterior part of the degenerated sublingua. 
 
 THYROID. 
 
 The thyroid arises primarily as a median ventral diverticulum 
 of the pharynx in the region of the first four or five visceral clefts, 
 and in the course of development may become subdivided into two 
 lobes. In addition to this unpaired diverticulum, paired portions, 
 situated more posteriorly, are developed in Mammals. 
 
 In the Ammoccete the single diverticulum, which is lined by a 
 ciliated epithelium, opens into the pharynx between the third and 
 fourth clefts (Fig. 221), but in the adult Petromyzon the organ, 
 as in all Vertebrates, loses its connection with the pharynx, under- 
 goes a modification, and gives rise to numerous closed glandular 
 vesicles enclosing an albuminous substance. 
 
 In Elasmobranchs the thyroid is unpaired and lies beneath the 
 mandibular symphysis; in adult Teleosts it is paired, and is 
 situated in the region of the first branchial arch. In Dipnoans it 
 lies anteriorly to the muscles of the visceral skeleton and shows an 
 indication of a division into right and left lobes. 
 
 In the Urodela and Anura the thyroid gives rise to numerous 
 vesicles situated close to the anterior end of the pericardium, 
 posteriorly to the second ceratobranchials in the former and on the 
 ventral side of the posterior cornua of the hyoid in the latter. 
 
 In Lizards it is usually situated close to the trachea (Fig. 
 204, A), and in Chelonians and Crocodiles it often possesses right 
 
256 
 
 COMPARATIVE ANATOMY 
 
 and left lobes lying on the great vessels just after they leave the 
 heart. In Birds (Fig. 205) the organ is paired, and lies close to 
 the origin of the carotid arteries. 
 
 The thyroid of Mammals consists of two lobes often connected 
 
 by a median isthmus, situated on 
 the ventral side of the larynx and 
 trachea (Fig. 190). 
 
 It appears probable that the 
 thyroid represents a very ancient 
 glandular organ, the secretory func- 
 tion of which in relation to the 
 alimentary canal was of great im- 
 portance in the ancestors of Verte- 
 brates. In existing forms it has 
 undergone a change of function, and 
 thus instead of disappearing, remains 
 as an important organ in the adult : 
 in Mammals especially it is charac- 
 terised by a great richness in blood- 
 vessels. What this function is, is not 
 thoroughly understood, but it has 
 been shown that its albuminous 
 secretion contains iodine, and is 
 passed into the blood- and lymph- 
 vessels ; and that extirpation of the 
 organ is followed by various distur- 
 bances of the mental and organic 
 functions. 
 
 The structure known as the ' ' carotid 
 gland " in Mammals, which is situated at 
 the bifurcation of the common carotid into 
 external and internal carotids, has not, as 
 was formerly supposed, anything to do with 
 the thyroid or thymus. It is abundantly 
 provided with nerve-cells. 
 
 THYMUS. 
 
 The thymus has always a paired 
 origin, and in the adult consists of 
 lymphoid tissue. In Elasmobranchs 
 it arises on either side from the epi- 
 thelium lining the upper part of 
 the first five gill-clefts, close to the 
 
 f all g lia of the ni " th and <** th cer f- 
 bral nerves, as well as in the neigh- 
 
 bourhood of the spiracle. The func- 
 
 tioa of f* <8i. >ogh doubtless 
 r, thyroid. a very important one, is not understood. 
 
 Tm 
 
 FIG. 205.-THYMUS AND THYROID 
 OF A YOUNG STORK. 
 
(ESOPHAGUS, STOMACH, AND INTESTINE 257 
 
 In Fishes and Dipnoans'the thyinus is more or less subdivided, 
 and is situated dorsally to the gill-arches. In Amphibians it 
 lies behind and above the articulation of the lower jaw, and in 
 Reptiles in the neighbourhood of the carotid artery, either close in 
 front of the heart (e.g., Snakes) or more anteriorly. In Birds, as 
 in young Crocodiles, it is elongated and more or less lobed, 
 extending all along the neck (Fig. 205). In Mammals the greater 
 part of the thymus is situated in the thorax, between the sternum 
 and heart, only a small portion extending into the neck. It is 
 largest in young animals, and usually becomes more or less 
 completely degenerated subsequently. 
 
 (ESOPHAGUS, STOMACH, AND INTESTINE. 
 
 Ichthyopsida. The oesophagus is short, and usually not 
 distinctly marked off from the stomach, though exceptions to this 
 rule often occur (e.g., many Teleostei, Siren lacertina Fig. 210, A). 
 The stomach is often defined as a widened section of the 
 enteric canal situated between the posterior end of the gullet 
 and the entrance of the bile duct. Such a dilatation cannot 
 accurately be spoken of as a stomach unless its epithelium 
 possesses specific characteristics and gives rise to gastric glands 
 (p. 267) ; in this sense a stomach is wanting in Amphioxus, Cyclo- 
 stomi, Holocephali, certain Teleostei (e.g., Cyprinidae, certain 
 Labridse, Gobiicla?, Bleniidae, Syngnathus acus, Cobitis fossilis), 
 and Dipnoi (Fig. 209). Whether this is a primitive character in 
 these forms or is due to degeneration is uncertain. 
 
 In other Fishes (Elasmobranchs, Ganoids, numerous Teleosts),. 
 as well as in all Amphibians, a true stomach is present, and is 
 usually externally recognisable as a more or less dilated sac; it 
 may be curved on itself, so as to form a U-shaped loop, the two 
 (cardiac and pyloric) limbs of which lie parallel to one another (Fig. 
 206). In general, its form is adapted to that of the body : thus 
 Rays and Anurans possess a far wider stomach than do most 
 other Fishes and Amphibians (comp. Figs. 206 210), and this rule 
 holds good also for Reptiles. The stomach of Teleosts varies con- 
 siderably in form. 1 
 
 The intestine may be straight or nearly straight, or may be 
 more or less coiled, and in the former case a spiral fold or valve 
 may be developed in Fishes, to increase the absorptive surface. 
 
 *In the Lamprey a longitudinal fold or typhlosole, taking a slightly 
 spiral course, extends into the lumen of the intestine. In Elasmo- 
 
 1 In numerous Teleosts (e.g., Tinea vulgaris, Cobitis fossilis) outer longi- 
 tudinal and inner circular striated fibres are present in both stomach and intestine 
 externally to the unstriated muscular coat. They grow] backwards from the 
 oesophagus. 
 
 S 
 
'258 
 
 COMPARATIVE ANATOMY 
 
 HH H 
 
CESOPHAGUS, STOMACH, AND INTESTINE 259 
 
 branchs, Ganoids, and Dipnoans, the fold 
 and forms a well-marked spiral valve, the 
 turns of which may lie so close together 
 as to almost fill the cavity of the intestine 
 (Figs. 206, 207, 209). In the Ganoids it 
 begins to undergo degeneration ; thus in 
 Lepidosteus (Fig. 207) it is only present 
 in the hinder part of the intestine. 
 Traces of a spiral valve can even be re- 
 cognised amongst the Teleostei (Cheiro- 
 centrus and possibly certain Salmonidse). 
 
 Pyloric cceca are met with in Ganoids 
 and numerous Teleosts, and consist of 
 longer or shorter finger-shaped processes 
 of the small intestine, situated posteriorly 
 to the pylorus in the region of the bile- 
 duct (Figs. 207 and 208). Their number 
 varies from 1 (Polypterus and Ammo- 
 dytes) to 191 (Scomber scomber). The 
 pyloric caeca and spiral valve seem to have 
 a similar function, and, as a general rule, 
 to be developed in inverse proportion to 
 one another. 
 
 In the narrow-bodied Gymnophiona 
 the intestine is only slightly coiled, while 
 in Anura it becomes considerably folded 
 on itself: its form in Salamanders is about 
 mid- way between these two extremes. 
 
 In the Cyclostomi, Holocephali, 
 Ganoidei, and most Teleostei, there is 
 a separate anus ; in all other Fishes as 
 well as in the Dipnoi and Amphibia the 
 large intestine opens into a cloaca common 
 to it and to the urinogenital ducts. The 
 large intestine (rectum) is comparatively 
 short and takes a straight course; in 
 Amphibians, as well as to some extent 
 in certain Ganoids and Teleosts, it is 
 plainly marked off from the small 
 intestine, and between the two there is 
 often a circular valve. In some cases 
 the rectum is considerably swollen and 
 may even exceed the stomach in cap- 
 acity (Fig. 210, B\ An outgrowth of 
 the ventral wall of the cloaca in Am- 
 phibia gives rise to the urinary Uadder, 
 and represents the allantow (p. 9) of 
 higher forms. 
 
 is more highly developed 
 -a.V 
 
 st 
 
 FIG. 207. ALIMENTARY VIS- 
 CERA AND AIR-BLADDER OF 
 Lepidosteus, in situ. (After 
 Balfour and Parker. ) 
 
 a, anus ; a. b, air-bladder ; 
 a. b ] , its aperture into the 
 throat ; b.d 1 , aperture of 
 bile-duct into intestine ; c, 
 pyloric cseca ; y.b, gall- 
 bladder ; hp.d, hepatic 
 duct ; lr, liver ; py, pyloric 
 valve ; *, spleen ; sp.v, spiral 
 valve ; st, stomach. 
 S 2 
 
A- 
 
 FIG. 208. 
 
 FIG. 209. 
 FIG. 208. ALIMENTARY CANAL or PERCH. 
 
 Oe, oesophagus ; M, stomach ; f, caecal process of latter ; P P, short pyloric 
 region ; Ap, pyloric caeca ; MD, small intestine ; ED, rectum ; A , anus. 
 
 FIG. 209. ALIMENTARY CANAL AND APPENDAGES OF Protopterus annectens. 
 
 (After W. N. Parker.) 
 
 as, oesophagus; st, "stomach"; py.v, pyloric valve; b.ent, bursa entiana 
 (anterior portion of intestine) ; ap.v, spiral valve ; re, rectum ; cl, cloaca ; 
 cLc, cloacal caecum ; v, vent ; a.p, abdominal pore ; ood, base of oviduct ; 
 k.d, base of kidney duct; Ir, liver; g.b, gall-bladder; h.d, hepatic ducts; 
 cy.d, cystic duct; b.d, common bile duct, and b.d 1 , its aperture into the 
 intestine ; c.m.a, coeliaco-mesenteric artery ; m.a", ra.a 3 , mesenteric arteries ; 
 h.p.v, hepatic portal vein ; sp, spleen. The pancreas is not seen, as it is 
 embedded in the walls of the "stomach" and anterior part of the intestine 
 on the dorsal and right side. 
 
CESOPHAGUS, STOMACH, AND INTESTINE 261 
 
 -Oe 
 
 D 
 
 FIG. 210A. FIG. 210B. 
 
 FIG. 210A. ALIMENTARY CANAL OF Siren lacertina. 
 
 Oe, oesophagus, marked off from the stomach (M) by a constriction, f ; P, pyloric 
 region ; MD, small intestine ; ED, large intestine. 
 
 FIG. 210B. ALIMENTARY CANAL OF Rana esculenta. 
 
 0e,"cesophagus ; M , stomach ; Py, pyloric region ; Du, duodenum ; D, ileum ; f, 
 boundary between the latter and the large intestine (R] ; A, opening of the 
 rectum into the cloaca (Cl) ; Hb, urinary bladder ; Mz, spleen. 
 
 In Elasmobranchs a finger-shaped rectal gland (processus digiti- 
 formis) opens into the anterior part of the rectum, and this perhaps 
 corresponds to the caecum of higher forms (see pp. 262, 266). Traces 
 
262 COMPARATIVE ANATOMY 
 
 of a caecum are seen in certain Teleosts. In the Dipnoi a cloacal 
 caecum is present (Fig. 209). 
 
 In all Fishes in which a cloaca is absent (p. 259) the anus is 
 anterior to the urinogenital aperture. 
 
 Reptiles. In correspondence with the more definitely differen- 
 tiated neck, the oesophagus of Reptiles is relatively longer than in 
 the animals as yet considered; it is always plainly marked off 
 from the much wider stomach, which is usually sac-like, or bent 
 upon itself, in which latter case it lies transversely (Chelonians). 1 
 As regards external form, the stomach of Crocodiles is more 
 specialised than that of other Reptiles, approaching that of Birds. 
 
 Snakes, Snake-like Lizards, and Amphisbaenians possess a 
 narrow, spindle-shaped stomach, which lies in the long axis of the 
 body ; in correspondence with the large size of the masses of food, 
 which are swallowed whole, it is capable of great distension. In 
 these the intestine is only slightly coiled : in Lizards the coils 
 are more marked, and in the other forms, with broad bodies, the 
 folding is carried still further. 
 
 The large intestine has a straight course, is often considerably 
 swollen, and opens into a cloaca. It may (e.g., certain Chelonians) 
 be as long as the small intestine and be bent on itself. An account 
 of the urinary (allantoic) bladder present in many Reptiles will be 
 found in a subsequent chapter. 
 
 From the Reptilia onwards a blind-gut or caecum is usually 
 formed at -the anterior portion of the large intestine : it is 
 generally asymmetrical. 
 
 Birds. In correspondence with the kind of nutriment, the 
 mode of life, and the absence of teeth, certain modifications of the 
 oesophagus and stomach occur in Birds. In graminivorous Birds and 
 Birds of Prey either the whole gullet forms a dilated sac or else it 
 gives rise to a ventral outgrowth ; in both cases the enlargement 
 is known as the crop (inglumes) (Fig. 211). This serves as a food 
 reservoir, and in some cases its walls are glandular. 
 
 The stomach, instead, of remaining simple, generally becomes 
 divided externally into two portions, an anterior and a posterior 
 (Fig. 211). The former, which on account of its richness in glands 
 is called the glandular stomach (proventricuhis\ alone takes part 
 in dissolving the food ; while the latter, which is lined by a horny 
 layer consisting of a hardened glandular secretion, has simply a 
 mechanical function, in correlation with which a peculiar and very 
 thick muscular wall provided with two tedirious discs is developed. 
 The degree of development of this muscular stomach, or gizzard, 
 stands in direct proportion to the consistency of the food. In 
 
 1 The oesophagus of marine Chelonians, 'like that of many Birds, is lined by 
 horny papillae. 
 
(ESOPHAGUS, STOMACH, AND INTESTINE 
 
 263 
 
 graminivorous Birds we find the strongest muscular layer and the 
 thickest horny lining, while in the series of insectivorous Birds, up 
 to the Birds of Prey, this condition becomes gradually less marked, 
 and the division of labour is less noticeable. 
 Thus in the series of existing Birds we can 
 trace the course of the phylogenetic differ- 
 entiation of the organ. 
 
 The small intestine is usually of consider- 
 able length and becomes folded on itself to 
 a greater or less degree ; it varies, however, 
 both in form, length, and diameter. 
 
 The straight large intestine opens into a 
 cloaca, and varies as to its relative diameter. 
 The caecum is usually paired, and may reach 
 a relatively enormous length (Lamellirostres, 
 Rasores, Ratitse). All kinds of intermediate 
 stages between this and an entire absence 
 of a caecum are to be met with. When 
 the caecum is largely developed, it has 
 an important relation to digestion, as an 
 increase of surface of the mucous membrane 
 is thus effected ; this increase may even be 
 
 carried further by each caecum being provided FIG. 211. -DIAGRAM or 
 with a spiral fold consisting of numerous 
 turns, as in the Ostrich. 
 
 The so-called bursa Falricii is a structure 
 peculiar to Birds, and arises as a small, 
 solid, epithelial outgrowth from the ectoder- 
 mtil portion of the cloaca, later becoming 
 excavated to form a vesicle. It is situated 
 in the pelvic cavity between the vertebral 
 column and the posterior portion of the intestine, and extends to 
 the outer section of the cloaca, into which it opens posteriorly to 
 the urinogenital ducts. It is probably present in all Birds, but 
 becomes atrophied more or less completely in the adult ; its physio- 
 logical function is quite unknown. 
 
 Mammals. The oesophagus, like that of Birds, is sharply 
 marked off from the stomach, and its muscles consist to a greater 
 or less extent of striated fibres : in Ruminants the latter extend as 
 far as the stomach. 
 
 The stomach undergoes much more numerous modifications 
 than are met with in any other Vertebrate Class. As a rule it 
 takes a more or less transverse position and has a sac-like form, 
 the cardiac portion, into which the oesophagus opens, being usually 
 more swollen and having thinner walls than the pyloric portion 
 which communicates with the duodenum. 
 
 According to the definition given on p. 257, a true stomach is 
 
 THE (ESOPHAGUS 
 AND STOMACH OF A 
 BIRD. 
 
 M, glan- 
 dular stomach; MM, 
 muscular stomach ; 
 MD, duodenum. 
 
261 
 
 COMPARATIVE ANATOMY 
 
 Oes 
 
 FIG. 212. DIAGRAMS OF THE STOMACH IN VARIOUS MAMMALS SHOWING THE 
 DIFFERENT REGIONS. (After Oppel.) 
 
 A, ORNITHORHYNCHUS ANATINUS ; B, KANGAROO (Doreopsis luctuosa) ; C, TOOTH KD 
 WHALE (Ziphius] ; D, PORPOISE ; E, HORSE ; F, PIG ; G, HARE ; H, HAM- 
 STER ( Cricetus frumentarivx). 
 
 (The oesophageal region (lined by stratified epithelium) is indicated by transverse 
 lines ; the region of the cardiac glands by oblique lines ; that of the fundus 
 glands by dots, and that of the pyloric glands by crosses. ) 
 
 Ocs, oesophagus ; P, pylorus ; D, duodenum ; / IV (in D), the four chambers of 
 the stomach ; I (in B), lymphoid tissue ; x...x (in B), boundary line between 
 the oesophageal and cardiac regions ; /(in H), fold bounding the <esophagenl 
 region. 
 
(ESOPHAGUS, STOMACH, AND INTESTINE 205 
 
 wanting in Monotremes (Fig. 212, A) : although the organ is 
 represented by a wide sac, it is entirely wanting in glands, and is 
 lined throughout by a stratified epithelium. This remarkable 
 condition is doubtless due to degeneration. Amongst Edentates, 
 a similar peculiarity is seen in Manis javanica in which, however, 
 the glands are retained in a sac-like outgrowth from the greater 
 curvature, and in Sloths in which the glands are more numerous. 
 
 In herbivorous Mammals the stomach is, as a rule, relatively 
 larger and more complicated than in carnivorous Mammals 
 
 FIG. 213. STOMACH or SHEEP. (From Oppel. After Carus and Otto.) 
 
 , oesophagus ; &. c, d, the three subdivisions of the paunch, marked off from one 
 another by the folds e and/; g, reticulum ; h, cesophageal groove ; i, psalte- 
 rium ; k, aperture leading from the psalterium into the abomasum (/, m) ; n, 
 pyloric valve ; o, duodenum. 
 
 (Figs. 212 and 213), and it may become divided into two or -more 
 chambers. In some Rodents and in the Horse distinct cardiac and 
 pyloric chambers can be recognised, and in Ungulates numerous 
 intermediate forms between a simple and an exceedingly complex 
 stomach, such as occurs in the typical Ruminants, are to be met 
 with. In the latter (Fig. 213) the stomach is divided into four 
 chambers, which are called respectively rumen (paunch), reticulum, 
 psalterium, and ahomasum. The two first, which may be looked 
 
266 COMPARATIVE ANATOMY 
 
 upon as parts of one and the same chamber, simply serve as storage 
 cavities, the food returning from them into the mouth, once more 
 to undergo mastication. It then passes into the psalterium, and 
 finally into the abomasum, the latter alone being provided with 
 gastric (rennet) glands, and serving as the true digestive stomach. 
 The psalterium is the latest to be differentiated both phylo- and 
 ontogenetically, and is rudimentary in the Tragulidse. In Camels 
 the rumen gives rise to two masses of gland-containing outgrowths, 
 known as " water-cells." In the Cetacea (Fig. 212, c and D), Hippo- 
 potamus, and Bradypus, the stomach is divided into several chambers, 
 and various other modifications in form and structure are met with 
 amongst Mammals. Thus in the Kangaroo, for instance (Fig. 212, 
 B), the walls of the stomach are curiously folded. 
 
 The small intestine is usually long, and varies more as to rela- 
 tive length and diameter in domesticated than in wild forms. 
 
 The large intestine, which is made up of a varying number 
 of coils, usually reaches a great length, and its diameter is 
 much greater than that of the small intestine : these two por- 
 tions are thus sharply marked off from one another, and the 
 distinction between them is rendered still more marked by the 
 sacculations of the anterior part of the large intestine. Only the 
 posterior portion of the latter, or rectum, which passes into the 
 pelvic cavity, corresponds to the large intestine of lower Verte- 
 brates ; the remaining and far larger part occurs only in Mammals, 
 and is called the colon. 
 
 The caBcum, which is almost always present, undergoes various 
 modifications both as to form and size. Thus in Edentates (Manis, 
 Bradypus), Carnivora, Odontoceti, Insectivora, and Cheiroptera, 
 it is very small or even entirely wanting, while in Herbivora 
 it may exceed the whole body in length. An inverse development 
 in size is usually noticeable between it and the rest of the large 
 intestine. In many cases (many Rodents, Monkeys, and Man) an 
 arrest of a portion of the cascum takes place in the course of 
 individual development, so that little more than the distal end 
 (processus vermiformis) remains (Fig. 190). In Lepus the enor- 
 mous caecum is provided with a spiral valve ; and in Hyrax, besides 
 a large sacculated caBcum at the junction of the small and large 
 intestines, there is a pair of large, simple, conical caeca further back. 
 
 Monotremes only amongst Mammals possess a distinct cloaca, 
 though in Marsupials and some Rodents the anal and urinogenital 
 apertures are surrounded by a common sphincter. In other 
 Mammals these apertures become completely separated from one 
 another. 
 
HISTOLOGY OF THE MUCOUS MEMBRANE 267 
 
 HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY 
 
 CANAL. 
 
 The epithelium lining the alimentary canal of Vertebrates with 
 the exception of that of the mouth and cloaca, which is usually 
 stratified consists primitively, that is, phylogenetically, of amoeboid 
 or cilated cells. In some cases this is also true ontogenetically, and 
 in Amphioxus and Protopterus, for instance, the ciliated epithelium 
 persists throughout life, and in the Ammocoete until metamorphosis. 
 In the adult Petromyzon, as well as in many Fishes and even Amphi- 
 bians, ciliated epithelium occurs constantly only in certain parts of 
 the gut, and in the higher Vertebrates cilia are only seen excep- 
 tionally after the embryonic period, so that, as a rule, only ordinary 
 columnar epithelium is present. A striated margin is observable 
 along the free border of the columnar cells (Fig. 214, A, B, a) ; this 
 is probably to be looked upon as the expression of the earlier 
 ciliated covering, and in some lower Vertebrates (e.g., Proteus and 
 Salamander larvae) it is capable of an active amoeboid movement 
 (B, 5). In this active participation of the cells in the process of 
 absorption, we recognise an old inheritance from primitive Inver- 
 tebrates (inlracellular digestion) ; but, at the same time, extra- 
 cellular or chemical digestion is the more important and occurs 
 exclusively in the higher types. 
 
 Numerous lymph-cells or leucocytes (p. 290) are present in great 
 numbers in the connective-tissue layer of the mucous membrane, 
 and often form definite masses or follicles (e.g., "Peyer's patches "). 
 In some cases (e.g., Protopterus), the development of this lymphatic 
 tissue along the gut is relatively enormous (cornp. p. 334). The 
 amoeboid leucocytes may even force their way into the lumen of 
 the intestine : a similar migration of these cells occurs in all 
 mucous membranes and in the walls of many vessels. 
 
 In Amphioxus, Cyclostomi, and Dipnoi, the whole of the 
 alimentary epithelium must be considered as secretoiy, each 
 individual cell acting as an independent gland. In Fishes, 
 Amphibians, and Reptiles, a higher stage is reached, inasmuch 
 as groups of cells in the stomach gives rise to tubular glands of a 
 simple nature (Fig-. 214). 1 A further differentation of the cells 
 leads to the condition seen in the gastric glands of Mammals, in 
 which the cells become differentiated into chief cells and parietal cells. 
 
 According to their position, three kinds of glands can be dis- 
 tinguished in the stomach of Mammals, viz., cardiac, fundus-, and 
 pyloric glands. The fundus glands have the greatest physiological 
 importance (comp. Fig. 212). 
 
 In the higher Vertebrates, more especially in Birds and 
 Mammals, the epithelium of the intestine also gives rise to tubular 
 
 1 Even in Ganoids and Teleosts the cells of the neck and fundus of each gland 
 are said to be different in character, pepsin being formed in the latter. 
 
268 
 
 COMPARATIVE ANATOMY 
 
 structures known as the crypts or glands of Licberkuhn. as well as 
 to acinous mucous glands. Mucous-secreting goblet cells are 
 
 BM M 1 Z IS 
 
 a 
 
 \ 
 
 FIG. 214. A, SEMIDIAGRAMMATIC TRANSVERSE SECTION or A PORTION OF THE 
 WALL OF THE INTESTINE. (Combined from the condition seen in both lower 
 and higher Vertebrates. ) B, Epithelial cells of the intestine. 
 
 /*, peritoneal investment of the gut ; M, longitudinal muscular layer ; M 1 , circular 
 muscular layer ; Z, connective-tissue layer ; S, mucous membrane, which is 
 raised to form villi at Zo. (The connective-tissue layer and epithelium are 
 designedly drawn much too large relatively as compared with the outer 
 coats.) G, G, vessels, the larger trunks running between the peritoneum 
 and the muscular layer ; the finer vessels branch out into the intermediate 
 layer ; these surround the masses of lymph-cells, LL, as well as the glands, 
 and send fine loops into the villi (at O l ) ; DD, apertures of the. glands ; 
 E, E, epithelial cells of the mucous membrane, with their striated border, 
 from which at E l amoeboid processes are extruded : in Fig. B, a, 6, these 
 cells are drawn to a much larger scale (Sa, striated border) ; Ly, scat- 
 tered lymph-cells in the intermediate layer ; L l , L*, lymph-cells in the act of 
 passing through the mucous membrane ; at L' : , several have already passed 
 into the alimentary cavity ; LL, masses of lymph-cells (solitary follicles) ; 
 Lym, lymph -vessels in the villi. 
 
 common throughout the alimentary epithelium of Vertebrates. 
 In Monotremes, as already mentioned (p. 265), gastric glands are 
 absent, but the intestinal glands are highly developed. 
 
LIVER 
 
 260 
 
 In order to effect an increase of the absorptive surface, longi- 
 tudinal folds of the mucous membrane are formed, and a special 
 development of such a fold, taking a spiral course, may result in 
 the formation of a spiral valve (see p. 257). A further advance is 
 seen in the development of transverse folds between the longi- 
 tudinal ones these are already seen in Elasmobranchs and many 
 other Fishes ; and by still further modifications, crypts of varied 
 
 ft ! ! 
 
 c 
 
 D 
 
 E 
 
 m 
 
 m 
 
 \V'x 
 
 FIG. 215. SEMIDIAGRAMMATIC FIGURES OF THE Mucous MEMBRANE OF THE 
 INTESTINE OF FISHES, SHOWING INTERMEDIATE FORMS BETWEEN LONGITU- 
 DINAL FOLDS AND ROUND CRYPTS. 
 
 A, Petromyzon, showing the distinct spiral fold ; B, an Elsmobranch C to E, 
 
 various Teleosts. 
 
 form and depth are produced, into which open the microscopic 
 glands, when present (comp. Fig. 215). 
 
 Finger-shaped outgrowths or villi of the mucous membrane of 
 the intestine are first plainly distinguishable in Amphibians (espe- 
 cially Anura) and are especially well developed in Mammals. 
 In addition to these, folds of varied forms, on the surface of which 
 the villi may be situated, occur from the Amphibia onwards : as 
 examples may be mentioned the valvuli conniventes of Mammals 
 and Birds. 
 
 LIVER. 
 
 The liver, the form of which is always closely adapted to that 
 of the surrounding parts, underlies to a greater or less extent the 
 ventral side of the intestinal tract, and is present in all the Craniata. 
 It arises as an outgrowth from the endodermic epithelium of the 
 intestine close to the junction of the latter with the stomach. 
 
 In Amphioxus a simple sac -like csecum is present in this 
 
A. 
 
 FIG. 216A. LIVER OF Rana esculenta. (From the ventral side.) 
 L, L 1 , L 2 , the different obes of the liver ; M, stomach ; Du, duodenum ; 
 
 H, heart. 
 
 FIG. 216B. PANCREAS AND LIVER OF FROG, TO SHOW THE ARRANGEMENT OF 
 
 THEIR DUCTS. 
 
 L, L 1 , L 3 , the lobes of the liver turned forwards ; G, gall-bladder ; Dcy, cystic 
 ducts, which, together with the hepatic ducts (Dh), form a network from 
 which three collecting ducts arise, and these unite to form the common bile- 
 duct (Dc) : the latter passes through the substance of the pancreas (P), 
 receiving further hepatic ducts (Dh 1 ), and the pancreatic ducts (P 1 ) ; at 
 Dc 1 it becomes free from the pancreas, and passes back to open into the 
 duodenum (Du) at Dc 2 ; Lhp, duodeno-hepatic omen turn ; M, stomach ; Py, 
 pylorus. 
 
LIVER 
 
 271 
 
 region which possibly represents the rudiment of the liver (Fig. 219). 
 In Craniates the outgrowth develops into a large vascular and 
 glandular organ, which gives rise to bile, and remains in communi- 
 cation with the intestine by means of one or more bile-ducts x 
 
 ED 
 
 FIG. 217. VISCERA OF Lacerta agilis. (From the ventral side.) 
 
 Oe, oesophagus ; M, stomach ; MD, small intestine ; ED, large intestine ; Z/, 
 liver ; GB, gall-bladder ; Pn, pancreas ; SI, urinary bladder ; Lg, Lg l , 
 the two lungs, with their network of vessels ; //, heart ; Ci, postcaval, Tr, 
 trachea. 
 
 ^Figs. 206, 209, and 216-218). It is united to the body- wall by a fold 
 of the peritoneum, and varies considerably in the number of its 
 lobes : in Mammals there may be as many as six or seven (e.g., Dog, 
 Weasel). We may nevertheless fix upon a ground-form consisting 
 
 1 The single-lobed liver of the Lamprey undergoes a histological retrogression 
 (fatty metamorphosis) after transformation. The tubuli disappear, the cells 
 become filled with fat, and the gall-bladder and bile-duct become atrophied. 
 
272 
 
 COMPARATIVE ANATOMY 
 
 of two lobes as the primary one in all Vertebrates. The liver of 
 the Anamnia is usually relatively larger than that of the Amniota. 
 
 Carnivorous (fat-eating) animals generally possess a larger liver than 
 Herbivores. 
 
 A gall-Madder 'is very commonly present partially imbedded in 
 the right primary lobe. It is connected with the system of hepatic 
 
 FIG. 218. A, B, C, VARIOUS MODIFICATIONS IN THE ARRANGEMENT OF THE 
 
 BILE-DUCTS. 
 
 D, duodenum ; Vf, gall-bladder ; c and .<?, cystic duct ; h, hepatic duct ; ch, 
 common bile-duct ; Ac, hepato-cystic duct ; he, hepato-enteric duct. 
 
 ducts by means of a cystic duct, thus forming a common bile-duct 
 which opens into the anterior part of the intestine (duodenum). 
 Some of the chief variations in the relative arrangement of these 
 ducts are shown in Fig. 218. 
 
 PANCREAS. 
 
 The pancreas arises from the proximal portion of the small in- 
 testine, near the liver, in the form of several (usually two dorsal 
 and one ventral) independent endodermic outgrowths : these may 
 become fused later, or certain of them may undergo degeneration. 
 Thus in the adult there may be a single duct, or several inde- 
 pendent ducts opening into the intestine (e.g., Birds, Crocodiles, 
 Emydse, and some Mammals) or in some cases into the bile-duct 
 (Fig. 216, B). Varying much in form and size, the pancreas early 
 gives rise to a band-shaped or more or less lobulated organ, usually 
 lying in the fold of the duodenum. In some cases it remains em- 
 bedded within the wall of the gut (e.g., Protopterus, 1 Fig. 209), and 
 amongst Teleosts (in which Order the pancreas was formerly sup- 
 posed to be absent in the adult) it may be surrounded by the liver 
 or have the form of scattered lobules extending throughout the 
 mesentery. 
 
 1 In Myxinoids a lobular gland is present in a similar relative position to the 
 pancreas of Protopterus in the neighbourhood of the bile-duct, into which it 
 opens : this probably corresponds to a pancreas. A dorsal pancreas is also de- 
 veloped in the embryo of Petromyzon, and a pancreas-like organ can be recognised 
 in the adult. The pancreas is not represented in Amphioxus. 
 
O. ORGANS OF RESPIRATION. 
 
 THE respiratory organs of Vertebrates are closely connected with 
 the alimentary canal both as regards position and development, and 
 are of two kinds, gills and lungs. 1 The former are phylogenetically 
 the older organs, and are adapted for aquatic respiration : they are 
 connected with the pharynx in the region of the visceral arches. The 
 latter always arise as sac-like outgrowths of the pharynx, which 
 grow backwards so as to lie within the body-cavity. 
 
 Both gills and lungs may be developed in the same individual, 
 but are usually not functional at the same time. They are sup- 
 plied with venous blood, which becomes purified while passing 
 through their capillaries. 
 
 The air-bladder or siuim-bladder present in many Fishes, and 
 acting as a hydrostatic organ (p. 280), arises in a similar manner to 
 the lungs that is, as an outgrowth from the fore-part of the ali- 
 mentary tract : it receives arterial blood from the aorta, and 
 venous blood passes from it ; but in some cases (e.g., Bony Ganoids 
 and a few Teleosts) it may act as an accessory respiratory organ. 
 
 I. GILLS. 
 
 The gills arise in connection with a series of laterally-arranged 
 outgrowths of the pharynx lying one behind the other, which be- 
 come open to the exterior. Passages or clefts are thus formed for 
 the water entering by the mouth, and in order that oxygen may 
 become absorbed, leaf-like or thread-like vascular processes, the 
 gills or branchiae, become developed in the region of each cleft. 
 These are internal or external, according to their position. 2 
 
 Fishes possess gills throughout life ; amongst Amphibians this 
 
 1 The integument (e.g., in Periophthalmus and Amphibia) and the intestine 
 {e.g., in Callichthys, Hypostomos, and Doras) may also take part in respiration. 
 
 2 External gills persist after hatching as functional respiratory organs only in 
 Polypterus, Calamoichthys, Protopterus, and the Amphibia, and in the Anura 
 they are soon replaced by internal branchiae (comp. pp. 278 280). 
 
 T 
 
274 
 
 COMPARATIVE ANATOMY 
 
 ^ .S *,3 
 
 * ": ihf 
 
 """S tJOr^T "> 
 
 U r ?^ C 
 
 o3 
 
 fe O 
 
 I r* 1 
 
 w -S3 
 
 = 
 
 *^*r 
 
 fl HI 
 
 I J|| 
 
 s 
 ^ .2 2 
 
 
 IT'S 
 
 * 1 
 c 1 
 
 'o C 
 
GILLS 275 
 
 is only the case in the Perennibranchiata : all the others simply 
 pass through a gilled stage, and later breathe by means of 
 lungs. Thus the study of this one Order furnishes us with an 
 excellent representation of the course of phylogenetic development 
 through which all the higher Vertebrates must have passed, and 
 which is still indicated in them by the appearance in the embryo 
 of gill-clefts and gill-arches with a corresponding arrangement of 
 the blood-vessels. These occur throughout the entire series of 
 the Amniota up to Man that is, in forms in which they are na 
 longer concerned in respiration. 
 
 Thus rudiments of five clefts are seen in the embryos of most Reptiles 
 and Birds, and of four in Mammals ; in many cases, however, they do not 
 become open to the exterior. Their order of disappearance is from behind 
 forwards, and the most anterior (hyoid) cleft persists in a modified condition 
 even in the adult, undergoing a change of function in connection with the 
 auditory organ (p. 224). Certain of the anterior arches persist in a modified 
 form (p. 69). 
 
 Amphioxus. The numerous (80 100, or more) gill-clefts of 
 Amphioxus, which are arranged in pairs and are supported by elastic 
 rods, extend backwards nearly to the middle of the body. At first 
 they open freely to the exterior, but at a later period of develop- 
 ment they become enclosed in an atrial or peribranchial chamber ,. 
 which opens by a single pore situated somewhat behind the middle 
 of the body (for details compare Fig. 219). 
 
 The relative extent of the branchial apparatus is considerably 
 limited even in the lowest Craniata. 
 
 Cyclostomes. In the larval Ammoccete the oesophagus is 
 continued directly backwards from the pharynx (Fig. 220, A), and 
 at the anterior end of the latter there is a muscular fold, the velum,. 
 covered by the mucous membrane (Fig. 221). 
 
 The seven gill-sacs provided with leaf-like folds of mucous mem- 
 brane which are present in the Ammocoete, persist in Petromyzon 
 but, with the formation of a suctorial mouth, the portion of the 
 oesophagus into which they open becomes closed posteriorly, and 
 the oesophagus apparently grows forwards above the latter, and 
 joins the mouth-cavity at the velum. Thus two canals pass back- 
 wards from the mouth, a ventral branchial or respiratory tube, and 
 dorsal oesophagus (Fig. 220, B). 
 
 In Petromyzon and Bdellostoma l the individual branchial sacs, 
 which communicate directly with the pharynx, open freely to the 
 exterior : in Myxine this original condition becomes modified by the 
 outer parts of the gill-passages growing out into long tubes, which 
 
 1 In Bdellostoma there are usually six or seven pairs of branchial sacs, and 
 behind these, on the left side, an cesophageo- cutaneous duct opens directly into the 
 pharynx, as is also the case in Myxine. Bdellostoma polytrema possesses thirteen 
 or fourteen pairs of gill-pouches. 
 
 T 2 
 
276 
 
 COMPARATIVE ANATOMY 
 
 unite to form a common duct on either side ; this opens far behind 
 the branchial apparatus on the ventral side of the body. 
 
 FIG. 220. DIAGRAM OF A LONGITUDINAL SECTION THROUGH THE HEAD OF 
 THE LARVAL (A) AND ADULT (B) Peiromyzon. 
 
 FIG. 221. LONGITUDINAL SECTION THROUGH THE HEAD OF AN Ammoccete. 
 
 V, velum ; P, papillae of mucous membrane ; K, K, K, three anterior gills ; Th, 
 thyroid (hypobranchial furrow) ; N, nasal sac ; *, communication be- 
 tween the ventricle of the olfactory lobe and that of the prosencephalon ; 
 Ep, epiphysis ; Jnf, infundibulum ; Hff, metencephalon ; Ml, medulla 
 oblongata ; b, c, ventricles of the mid- and hind-brain ; o, subdural cavity ; 
 Ch, notochord ; R, spinal cord. 
 
 Fishes. From the Elasmobranchii onwards, the gills are in 
 close relation with the visceral skeleton, and in these Fishes they 
 consist of closely-approximated transverse laminae (Figs. 222 A, 223), 
 which are firmly attached to both sides of the septa which separate 
 the individual gill-sacs from one another, so that each septum bears a 
 half-gill, or hemibranch, on both its anterior and posterior surface. 
 A gill, or holobranch, thus consists of the branchial arch phis the 
 posterior hemibranch of the sac in front of it and the anterior 
 hemibranch of the following sac. The gill-sacs, of which there are 
 commonly five, 1 open separately to the exterior, and a rudimentary 
 
 1 There are six in Hexanchus and Chlamydoselache and seven in Heptanchus. 
 
GILLS 
 
 277 
 
 gill-cleft known as the spiracle (p. 75), is as a rule present more 
 anteriorly, between the mandibular and hyoid arches. In the Holo- 
 cephali, however, the spiracle becomes reduced, there are only three 
 holobranchs in addition to hemibranchs on the hyoid and fourth 
 
 FIG. 222. Dissection of the head from the ventral side of A, an Elasmobranch 
 (Zygfuna malleus), and B, a Teleost (Gadus aeglefinus), to show the branchial 
 apparatus. In both figures the branchial arches on the left side are shown 
 cut through horizontally. (From R. Hertwig's Zoology.) 
 
 Pq, palatoquadrate, and a, its connection with the cranium anteriorly; uk, 
 lower jaw ; m, oral cavity ; prm, prernaxilla ; ma, maxilla ; pa, palatine ; 
 Jim, hyomandibular ; is, internal branchial apertures ; as, external branchial 
 apertures ; ops, opercular aperture ; h, branchial septum; ft/ 1 , anterior, and ft/ 2 , 
 posterior hemibranch of a gill-pouch ; op, operculum ; s, pectoral arch ; 2, 
 tongue ; phi, inferior pharyngeal bone ; o, oesophagus. 
 
 branchial arch, and an opercular membrane is present, covering the 
 external branchial apertures and opening by a slit posteriorly ; 
 traces of a similar structure are seen in Chlamydoselache. 
 
 In Ganoids and Teleosts there are no longer chambered gill- 
 sacs. The septa on which the gili-laminse are borne become 
 greatly reduced, so that the apices of the latter extend freely out- 
 wards ; the whole branchial region is, moreover, covered over by the 
 operculum and branchiostegal membrane (cornp. pp. 75 and 79), 
 
278 
 
 COMPARATIVE ANATOMY 
 
 and thus, as in the Holocephali, the gill-slits open into a common 
 branchial chamber, which communicates with the exterior by a 
 single slit-like aperture on either side (Figs. 222 B and 223). A 
 
 spiracle is present in Acipenser, Polyodon, 
 and Polypterus amongst Ganoids. 
 
 As a rule Teleosts possess only four holo- 
 braiichs, 1 and this holds good for all Ganoids. 
 
 A rudimentary gill or pseudobranch is present 
 011 the anterior wall of the spiracle of many 
 Elasmobranchs and of cartilaginous Ganoids 
 (mandibular pseudobranch) ; and the posterior 
 hyoid hemibranch, which is functional in Aci- 
 peiiser and Lepidosteus, becomes more or less 
 reduced in Ganoids and Teleosts, forming 
 the so-called opercular pseudobranch. Traces of 
 a cleft, lying behind the functional branchial 
 clefts, are found in the embryos of certain 
 Elasmobranchs. All these facts indicate that 
 Fishes formerly possessed a more extensive 
 branchial apparatus than at present. 
 
 In the Lophobranchii the gills are replaced 
 by tufted processes, and in many Teleostei 
 
 HOLOBRANCH OF Zygoe.ua certain accessory structures are developed in 
 (ON THE RIGHT) AND the region of the branchial chamber by a modi- 
 Gadus (ON THE LEFT), fication of the branchial arches or cavities. 
 SLIGHTLY ENLARGED. These serve to retain the water, and thus the 
 (From R. Hertwig's Fish is able to live for some time out of the 
 water (Anabas, Saccobraiichus, Heterobranchus, 
 Clarias). 
 
 External gills are met with in young stages 
 of Elasmobranchii and Holooephali as well as 
 in Polypterus and Calamoichthys ; in Elasmo- 
 branchii and Holocephali, at any rate, they are 
 endodermal and not ectodermal in origin. 
 
 FIG. 223. TRANSVERSE 
 SECTION THROUGH A 
 
 Zoology). 
 
 b, branchial arch ; z, gill- 
 rakers ; a, afferent, 
 and v, efferent branchial 
 vessels ; bl 1 , anterior, 
 and bl 2 , posterior hemi- 
 branch of the gill ; r, 
 cartilaginous gill -ray; h, 
 septum. 
 
 Fishes breathe by taking in water 
 through the mouth, and, by the con- 
 traction of the latter, forcing it out again through the gill- 
 slits. 2 In this process the' gill-arches rise and fall, separating 
 from one another during inspiration, and approximating during 
 expiration. 
 
 Dipnoi. These, as their name implies, possess both gills and 
 lungs, only the latter organs being functional in Protopterus during 
 its summer sleep (see p. 17). Besides the internal gills, which 
 are covered by a small operculum, Protopterus possesses three pairs 
 of external gills situated just above the operculum and supplied by 
 vessels from the arterial arches. In Ceratodus, in which, as in 
 Lepidosiren, no external gills are present, there are four complete 
 gills on the first four branchial arches, as well as a pseudobranch 
 
 1 They may be reduced to three, or two, and even these may become more or 
 less rudimentary. 
 
 2 In the Lamprey inspiration as well as expiration takes place through the 
 gill apertures when the animal is attached by means of its suctorial mouth. 
 
GILLS 
 
 279 
 
 on the hyoid. In Protopterus and Lepidosiren a reduction of these 
 organs has taken place, gills being absent in the former genus on 
 the first and second branchial arches ; there is, however, in addition, 
 an anterior hemibranch on the fifth branchial arch. 
 
 Amphibia. In the embryos of Urodeles, five gill-clefts can 
 usually be recognised, but the most anterior and posterior of these 
 do not become open to the exterior. In the larvaB, as well as in adult 
 Perennibranchiates, there are three external gill-tufts in connection 
 with the three anterior branchial arches, lying one over the other ; 
 
 - ssa^ggggsss 
 
 FIG. 224, A and B. LARVA OF Epicrium glvtinosum, WITH EXTERNAL GILLS. 
 
 (After Sarasin.) 
 
 these extend backwards, projecting freely to the exterior, and are 
 composed of connective-tissue, unsupported by cartilage. They 
 either have the form of tufts, or may be delicately branched, 
 showing the most varied arrangements for increasing the respira- 
 tory surface (comp. Fig. 224). These external gills are ectodermal 
 in origin, and must not be confused with the internal gills, which 
 are wanting in all Urodeles. They are acted on by a complicated 
 system of muscles, and are covered by ciliated epithelium, which 
 serves to keep up a continual current in the surrounding medium. 
 In the Axolotl and in larval Salamanders there are four, and in 
 Necturus (Menobranchus) and Proteus only two gill-clefts perfora- 
 
280 COMPARATIVE ANATOMY 
 
 ting the pharynx. The former thus show a more primitive con- 
 dition, while in the latter these structures have become reduced. 
 There is always only a single external opening covered over by an 
 opercular-like fold of skin. 
 
 The usually feather-like external gills present at first in Anura 
 soon disappear, and their place is taken by internal gills, the epi- 
 thelium covering which is also said to be ectodermal in origin. By 
 the growth of the opercular folds, which contain no skeletal parts, 
 the external respiratory aperture of either side becomes gradually 
 reduced in size, and the two branchial chambers usually open 
 eventually by a single aperture, which is situated either in the 
 median ventral line, or laterally. 
 
 The larvae of the Gymnophiona also possess external gills, which 
 vary much in form in the different genera (Fig. 224). 
 
 In certain Batrachia in which there is no free larval stage it appears 
 that respiration may take place before hatching in the broad and vascular 
 tail (Hylodes martinicensis), in folds of the ventral body wall (Rana 
 opisthodon), or in peculiarly modified external gills (Nototrema^). 
 
 Except in the Perennibranchiata, the gills disappear at meta- 
 morphosis and the respiratory apertures close up. In the Derotre- 
 mata, however, the gill cleft between the third and fourth branchial 
 arches persists. 
 
 II. AIR-BLADDER AND LUNGS. 
 1. THE AIR-BLADDER. 
 
 As already mentioned (p. 273), the lungs and swim-bladder are 
 developed in a similar manner, and only differ from one another in 
 the fact that the lungs always arise from the ventral side of the 
 pharynx, while this is only exceptionally the case as regards the 
 air-bladder (e.g., Polypterus, Calamoichthys), which usually arises 
 on the dorsal side. The exact point of origin of the air- 
 bladder from the alimentary canal varies, 1 and its duct (ductus 
 pneumaticus) may either remain open throughout life, as in all 
 Ganoids and some Teleosts (Physostomi), or it may later become 
 reduced to a solid fibrous cord or even entirely obliterated, as in 
 other Teleosts (Physoclisti). In the latter case there is no com- 
 munication between the swim-bladder and the external air, and it is 
 probable that the contained gas is given off from the walls of the 
 swim-bladder itself. A vascular organ (the so-called " rete mirabile "),. 
 consisting of numerous glands and capillaries, is present in the walls 
 of the swim-bladder in the Physoclisti, and in certain Physostomi a 
 
 1 In Erythrinus it arises laterally, and in some Physostomi (e.y., Herring) it 
 opens further back into the stomach. 
 
THE AIR-BLADDER AND LUNGS 
 
 281 
 
 somewhat similar organ ("red-body") is present, but consists of 
 capillaries only. 
 
 The air-bladder lies above the peritoneum on the dorsal side of 
 the body-cavity, between the vertebral column, aorta, and kidneys 
 on the one hand, and the alimentary canal on the other : it is 
 invested by the peritoneum on the ventral side only. It is more 
 or less sac-like in form, and is 
 only exceptionally paired (Poly- 
 pterus) ; it usually extends along 
 the whole length of the body- 
 cavity, and its walls are composed 
 of connective, elastic, and muscular 
 tissue. In some Teleostei it is 
 transversely constricted so as to 
 form several successive divisions ; 
 in other cases it may give rise to 
 a more or less numerous series 
 of csf3cal processes. 1 Its internal 
 surface may be either smooth or 
 spongy (Fig. 225) owing to the 
 formation of a meshwork of 
 trabeculse, the structure of which 
 resembles that of the lungs of 
 
 Dipnoi and Amphibia, and as already stated, it has a respiratory 
 function in some cases. 
 
 An air-bladder is wanting in Cyclostomes and Elasmobranchs. 
 
 Attention has already been directed to the relations which often 
 exist between the air-bladder and the auditory organ (see p. 226). 
 
 FIG. 225. INTERNAL SURFACE OF THE 
 AIR-BLADDER OF LEPIDOSTEUS* 
 SHOWING THE TRABECUL^;. 
 
 B, fibrous longitudinal band. 
 
 2. THE LUNGS. 
 
 The lungs arise at the hinder border of the branchial region of 
 the pharynx, which here becomes divided by a longitudinal hori- 
 zontal fold into a dorsal and a ventral portion, the latter of which 
 gives rise to a blind sac, opening anteriorly by a wide aperture into 
 the former and composed of endoderrn surrounded by mesoderm (Fig. 
 226). A longitudinal vertical furrow is then formed, dividing this 
 primitive lung-sac into right and left halves : the narrower proximal 
 portions of these represent the primitive bronchi, which communicate 
 with the pharynx by a single tube, the primitive windpipe or 
 trachea. The proximal end of the latter usually becomes differen- 
 tiated to form a larynx, or organ of voice, which opens into 
 the pharynx on its ventral side by means of a slit-like aperture, the 
 glottis. The lungs are therefore phylogenetically older organs than 
 
 1 In the Gymnodonts (e.g., Diodon, Tetrodon), the whole oesophagus is capable 
 of great distension. 
 
282 
 
 COMPARATIVE ANATOMY 
 
 the bronchi, trachea, and larynx, and this statement is supported by 
 a study of their comparative anatomy. 
 
 PD 
 
 PD 
 
 FIG. 226. A, B, C, DIAGRAMS SHOWING THE MODE OF DEVELOPMENT OF THE 
 
 LUNGS. 
 
 PD, primitive alimentary tube ; S, S l , the lung-sacs, which are at first unpaired ; 
 t, trachea ; b, bronchus. 
 
 FIG. 227. DIAGRAM ILLUSTRATING THE PHYLOGENETIC DEVELOPMENT OF THE 
 LUNGS ; A GRADUAL INCREASE OF THE RESPIRATORY SURFACE is SEEN IN 
 
 PASSING FROM A TO D. 
 
 Hollow outgrowths and buds arise from the endoderm of the 
 lungs and extend into the surrounding vascular mesoderm, which 
 
 FIG. 228. DIAGRAM OF THE EMBRYONIC HUMAN LUNG. (After W. His.) 
 
 Ap, pulmonary artery ; lr, air-passage ; sp, oesophagus ; lb, pulmonary vesicle 
 undergoing division ; O, right upper (anterior) lobe of the lung with its 
 eparterial bronchus ; M, V, middle and lower (posterior) lobes ; O 1 , left upper 
 lobe with its hyparterial bronchus ; F 1 , left lower lobe. 
 
 gives rise to muscular fibres and connective-tissue, and thus a 
 branched system of cavities communicating with the bronchi is 
 
AIR-TUBES AND LARYNX 283 
 
 gradually formed (secondary and tertiary bronchi). The ends of 
 these branches are swollen, forming vesicles known as infundibida, 
 which are made up of a number of alveoli, and are surrounded by 
 blood capillaries, through the thin walls of which the interchange 
 of respiratory gases takes place (Figs. 227 and 228). 
 
 In the following account the air- tubes will be dealt with 
 separately from the lungs proper. 
 
 Air-Tubes and Larynx. 
 
 The walls of the air-tubes may consist, in addition to their 
 lining of ciliated epithelium, of connective-tissue and elastic and 
 muscular fibres only, but usually cartilaginous elements are also 
 formed, and these serve to keep the tubes permanently open. The 
 most anterior of these cartilages, which support the larynx, become 
 differentiated to form a frame on which the structures by means of 
 which the voice is produced the vocal cords, are stretched : these 
 cartilages are acted upon by muscles. The relative length of the 
 windpipe, as a rule, corresponds with that of the neck. 
 
 Dipnoi. In these the glottis is supported by a fibre-cartilage, 
 and leads into a muscular vestibule communicating with the lung. 
 A larynx and trachea are not differentiated. 
 
 Amphibia. The vestibule, or laryngo-tracheal chamber, com- 
 municates with the pharynx on the one hand and with the lungs on 
 the other, and is supported by cartilages : it is provided with 
 intrinsic (dilator and constrictor) and extrinsic muscles, the former 
 derived from pharyngeal muscles and the latter from trunk muscles. 
 A definite trachea is differentiated in Siren, Amphiuma, and the 
 Gymnophiona only ; it reaches a length of 4 to 5 or more centi- 
 metres, and its wall is strengthened by a series of small irregular 
 cartilages, which usually tend to unite into bands (Fig. 229) : 
 only in the Gymnophiona, however, do these bands begin to take 
 on the form of half-rings, and to surround the trachea more or less 
 completely. 
 
 The phyletically oldest skeletal parts are a pair of arytenoid 
 cartilages, situated in the walls of the vestibule (Fig. 229) : these 
 appear to have arisen by a modification of part of the fifth bran- 
 chial arch (comp. Fig. 233). Distally to them there is, in the 
 Anura, another cartilage corresponding to the cricoid of higher 
 forms, and traces of this also occur amongst Urodeles (e.g., Siren). 
 
 In Anura a highly differentiated larynx is present. This is 
 regulated by a well-developed series of muscles, and is provided 
 with vocal cords, the sound produced by which is often intensified 
 by the presence of vocal sacs developed from the floor of the mouth. 
 The laryngo-tracheal chamber lies between the posterior cornua of 
 the hyoid (thyro-hyals) and is supported by a thin arytenoid cartilage 
 
284 
 
 COMPARATIVE ANATOMY 
 
 on either side of the glottis as well as by a ring-shaped cricoid 
 cartilage, from which delicate processes pass backwards to the 
 
 FIG. 229. LARYNGEAL AND TRACHEAL SKELETON OF UROPELES. A, Ntcturus 
 (Menobranchus) ; j3, Siren lacertina ; C, Amphiuma ; D, Salamandra maculosa. 
 
 a, the cartilages (arytenoids) on either side of the glottis ; a', ridge for muscles ; 
 *, the representative of the cricoid cartilage ; ft, cartilages of the trachea 
 in Siren ; Kb, the more definite tracheal cartilaginous tracts in Amphiuma 
 and Salamandra ; X IV , fourth branchial arch, from which the dilator (d) of 
 the trachea and larynx arises ; co, constrictor of the larynx ; L, L', lungs. 
 
 roots of the lungs (Fig. 230). Vocal cords are developed in the 
 Anura only, each being attached to the inner concave surface 
 of the corresponding arytenoid. 
 
 Ca 
 
 Sp 
 
 C.I* 
 
 FIG. 230. CARTILAGINOUS SKELETON OF THE LARYNGO-TRACHEAL CHAMBER OF 
 Rana esculenta. (A, from above ; B, from the side.) 
 
 Ca, Ca, arytenoid cartilage ; C.I 1 to C.I 4 , cricoid cartilage ; Sp, process of the 
 latter ; P, plate-like broadening out of the ventral part of the cricoid ; SR, 
 glottis ; ***, three tooth-like prominences of the arytenoids. 
 
 Reptiles. The larynx of Reptiles is supported by cartilaginous 
 elements comparable to those of Anura, there being two sets of 
 
AIR-TUBES AND LARYNX 
 
 285 
 
 cartilages a paired arytenoid, and a ring-shaped cricoid (Figs. 76 
 and 231). No considerable advance in structure is seen ; there is 
 even a reduction noticeable as regards the musculature as compared 
 with the Anura. 
 
 One point, however, must be specially noticed, viz., the close 
 connection which obtains between the larynx and the hyoidean 
 
 FIG. 231. LARYNX or Phyllodactylus eiLropceus. (A, skeleton, and B, 
 musculature of larynx.) 
 
 Ar, arytenoids ; Cc, cricoid ; S, anterior median process of cricoid ; S 1 , sphincter ; 
 D, dilator ; T, trachea ; Oe, basi-hyal. 
 
 apparatus more particularly the dorsal surface of the basi-hyal. 
 In Crocodiles and Chelonians, for instance, the larynx is firmly em- 
 bedded in a shallow depression of the latter (Fig. 76). 
 
 A well-developed trachea, supported by cartilages, is present in 
 all Reptiles ; but the cartilages are not in all cases fused together 
 to form complete rings. The walls of the bronchi are also usually 
 provided with cartilaginous supports. 
 
 Birds. In Birds there are two larynges, an upper and a lower. 
 The former lies in the usual position behind the tongue on the floor 
 of the pharynx, and is plainly homologous with that of other 
 Vertebrates, though it has become rudimentary and is incapable of 
 producing sound. 
 
 The lower larynx, or syrinx, is of much greater importance ; it is 
 usually situated at the junction of the trachea and bronchi, or more 
 seldom at the lower end of the trachea alone or on the bronchi 
 alone. It functions as the organ of voice, and appears first in, and 
 is restricted to, Birds. In the most usual form (broncho-tracheal 
 syrinx), there is a movable connection between the most anterior 
 bronchial rings, with which a complicated system of muscles is con- 
 nected ; these, by their contraction, cause a stretching or relaxing of 
 
286 COMPARATIVE ANATOMY 
 
 certain vibratory membranes. A bar of cartilage or bone, the 
 vessulus, extends from the junction of the bronchi into the more or 
 less swollen " tympanum " at the base of the trachea : this supports 
 a slight fold of the mucous membrane called the membrana 
 semilunaris, while the membranous inner wall of each bronchus is 
 known as the membrana tympaniformis internet, : the external wall 
 may also give rise to a membrana tympaniformis cxtcrna. The 
 
 Tr 
 
 FIG. 232. LARYNX OF MALE DUCK. (A, external, and B, internal view.) 
 
 Tr, trachea; Br, bronchus; T, the "tympanum"; S, pessulus, from which 
 a lateral outgrowth (8 between & and 6) extends into the tympanum, 
 thus dividing its aperture into the trachea into two portions (6, 6) ; the 
 aperture is further diminished by the circular fold of mucous membrane, SF ; 
 t, thin region in S. 
 
 tympanum attains a relatively enormous development in some 
 Water-Birds (e.g., the male Duck), where it gives rise to a bony 
 vesicle which serves as a resonance cavity (Fig. 232). 
 
 The length of the trachea in Birds varies greatly, and its complete cartila- 
 ginous rings usually become ossified. In some cases (e.g., the Swan and Crane) 
 it extends into the hollow keel of the sternum, where it becomes more or less 
 coiled, and then again passes out close to its point of entrance and enters the 
 body-cavity. In certain representatives of the SturnicUe it extends between 
 the skin and the muscles of the thorax, and there gives rise to numerous 
 spiral coils. 
 
 Mammals. The larynx of Mammals is distinguished from 
 that of all other Vertebrates by the marked differentiation of the 
 muscles the constrictors always exceeding the dilators in number 
 and by the constant presence of an epiglottis and a thyroid 
 cartilage. 
 
 The thyroid cartilage is derived from part of the fourth and 
 fifth branchial arches (comp. Fig. 233), and in Monotremes, in which 
 it is paired, it is still closely connected with the hyoid apparatus 
 
AIR-TUBES AND LARYNX 287 
 
 (comp. p. 285) . In all other Mammals the thyroid is impaired , though 
 still showing traces of its primary paired nature, and it becomes 
 
 I.. L 
 
 Ill 
 
 V 
 
 tr. 
 
 FIG. 233. DIAGRAM TO ILLUSTRATE THE METAMORPHOSIS DURING DEVELOP- 
 MENT OF THE FIRST TO FIFTH VISCERAL SKELETAL ARCHES (I V) IN 
 MAN. 
 
 From the proximal end of the first arch (Meckel's cartilage) two of the auditory 
 ossicles, the malleus and incus (mb and in) are represented as arising, p, pinna - T 
 pr, mastoid process of skull. 
 
 From the second arch (hyoid) arise proximally the styloid process (p.s), distally 
 the anterior (lesser) cornu of the hyoid (c.a) and a portion of the basi-hyoid (b.s). 
 By far the greater portion of this arch becomes the stylo-hyoid ligament (l.g). 
 (Concerning the stapes (*t] comp., p 101). 
 
 The third (first branchial) arch gives rise to the greater part of the body (b.s) 
 and the posterior or greater cornu of the hyoid (c.p. ). 
 
 The fourth (second branchial) arch gives rise to the upper segment (th f ) of the 
 thyroid cartilage, and the fifth (third branchial) to the lower one (th' 1 ). The 
 arytenoid cartilage (ar) is probably a derivative of the fifth arch, tc, cartilago 
 triticea ; cr, cricoid cartilage ; tr, trachea. 
 
 separated from the hyoid : it is shield-shaped, and surrounds the 
 lateral and ventral regions of the larynx, overlappiug the cricoid 
 
288 COMPARATIVE ANATOMY 
 
 above, 1 and serving as a point of origin and insertion for important 
 intrinsic and extrinsic muscles. 
 
 The vocal cords extend between the thyroid and the arytenoids, 
 and the mucous membrane above them becomes involuted to form 
 the laryngeal pouches. In Anthropoids and certain other Monkeys 
 (e.g., Mycetes) these may reach such a large size that they serve 
 as resonance cavities, and lie partially within the body of the 
 hyoid, which is swollen to form a large bony chamber (Fig. 234). 
 The folds of mucous membrane bounding the laryngeal pouches 
 anteriorly are spoken of as false vocal cords ; these are not present 
 in all Mammals. 
 
 The epiglottis, which consists of elastic fibro-cartilage, stands in 
 close relation to the soft palate, extending upwards from the anterior 
 border of the larynx, in front of the glottis : it is often, when at rest, 
 embraced more or Jess firmly by the soft palate in such a way 
 that its distal end lies in the passage of the posterior nostrils (naso- 
 pharyngeal chamber), so that respiration and feeding can go on 
 independently of one another. 2 
 
 An interesting adaptation for the method of lactation is seen in the 
 larynx of Marsupial embryos, in which it, together with the epiglottis, 
 becomes greatly elongated and is firmly embraced by the soft palate, so that 
 it cannot be moved from this position. Thus respiration can go 011 freely 
 while the milk passes down the oesophagus on either side of the larynx. 
 In Cetecea (e.g., Phocsena), a similar arrangement occurs, and is here 
 adapted for the aquatic life of the animal. Probably in all Mammals a 
 similar position of the larynx is seen in the embryo. 
 
 The Lungs proper. 
 
 Dipnoi. In Ceratodus the lung is a wide unpaired sac, without 
 any trace of a dividing septum : in other Dipnoans it is dis- 
 tinctly paired throughout the greater part of its length, the anterior 
 unpaired portion being largely filled up by spongy trabeculse. 
 
 The lung extends through the whole length of the body-cavity, 
 and is covered by peritoneum on the ventral surface only ; the 
 lining mucous membrane forms bands and networks similar to those 
 seen in the air-bladder of many Fishes (e.g., Lepidosteus, Fig. 225). 
 
 Amphibia. The lungs of Proteus and Necturus (Fig 235), 
 though paired throughout, remain at a lower stage than those 
 of the Dipnoi, inasmuch as their internal surface is perfectly smooth, 
 and has, therefore, a much smaller superficial extent. They 
 
 1 The cricoid may be complete or incomplete ventrally, and its dorsal portion 
 usually becomes raised to form a broad plate with which the arytenoids are articu- 
 lated (Figs. 233 and 234). 
 
 2 The epiglottis was probably originally a paired structure, consisting of 
 hyaline cartilage, and it is possible that the small cartilages of Wrisberg and 
 Santorini present in the larynx in addition to the more important cartilages de- 
 scribed above may be specialisations of part of the same structure. 
 
LUNGS 
 
 289 
 
 JSp- 
 
 Ct 
 
 B 
 
 Tr 
 
 FIG. 234. LARYXGES OF VARIOUS MAMMALS. 
 
 A, larynx of Deer, seen from the left side ; B, longitudinal section through the 
 larynx of the Fox ; C, larynx of the Howling Monkey (Mycetes ursiniw), from 
 the left side ; D, Larynx of Chipanzee (Simia troglodytes), from the ventral 
 side. 
 
 Tr, trachea ; Ctr, cartilaginous rings of the trachea ; S, mucous membrane of the 
 trachea and tongue ; Cr, ventral, and O 1 , dorsal plate of the cricoid ; Ct, 
 Ct l , thyroid cartilage ; oh, uh, anterior and posterior cornua of the latter ; 
 Ca, arytenoid cartilage ; pm, processus muscularis of the latter ; Ep, epi- 
 glottis ; H. body of hyoid ; h, lesser, h 1 , greater cornua of the hyoid ; Lt, 
 crico-thyroid ligament ; Mth, thyro-hj'oid ligament ; M, laryngeal pouch, 
 which shows an enlargement at t ; 1, 2, 3, the three resonance cavities of 
 Simia troglodytes ; mil, submucous tissue with muscles ; M.ge, genioglossus 
 muscle ; Z, tongue. 
 
290 
 
 COMPARATIVE ANATOMY 
 
 consist of two delicate elongated sacs of unequal length, and con- 
 stricted in the middle ; in Proteus they extend much further 
 backwards than in Necturus. A difference in length between 
 
 the two lungs is seen also in other Am- 
 phibia, such as Amphiuma and Siren, in 
 which the two cylindrical lungs lie near 
 together, close to the aorta. Their in- 
 ternal surface is raised into a network, 
 corresponding with the distribution of 
 the blood-vessels, the meshes being 
 much finer in Amphiuma, and still more 
 so in Menopoma, than in Siren. 
 
 In many Salamanders (e.g., Salaman- 
 drinse, Amblystomatinse, Desmognathinse, 
 Plethodontinse) the lungs undergo a more 
 or less complete degeneration, even though 
 all traces of the gills disappear. The fact 
 that the floor of the mouth is continually 
 raised and lowered as in other Amphibians 
 which possess lungs, and that in some 
 cases, at any rate, the animal dies if these 
 \\ ^ respiratory movements are prevented, in- 
 
 1 Jf dicates that a bucco-pharyngeal respiration 
 
 I/ takes place, and that cutaneous respiration 
 
 (which occurs in most Amphibians) alone 
 is insufficient. In other Salamanders the 
 lungs are as a rule equal in size, and have 
 the form of cylindrical tubes extending 
 backwards as far as the end of the stomach ; 
 their internal surface is more or less 
 smooth. The lungs of the Gymnophiona are similar to those of 
 Salamanders, but the right alone is fully developed, and this 
 shows in its interior a complicated trabecular network : the left 
 is only a few millimetres long. 
 
 The sac-like lungs of Anura are quite symmetrical. Their 
 internal surface, which is lined partly by ciliated epithelium,, 
 is raised up into a rich respiratory network of trabeculae, and 
 numerous smooth muscular fibres are present in their walls. 
 
 Reptiles. In Reptiles, as in all other air-breathing Verte- 
 brates, the form of the lungs is to a great extent regulated by 
 that of the body. In the higher types, such as the Chelonia and 
 Crocodilia, their structure is much more complicated than in 
 Amphibia ; this complication finds expression in a very considerable 
 increase of the respiratory surface. With the exception of the 
 thin -walled lungs of Lizards, which retain a very primitive con- 
 dition, we no longer meet with a large central cavity, but the 
 organ becomes penetrated by a branched system of bronchi, which 
 
 FIG. 235. LUNGS OF PRO- 
 TEUS (A) AND NECTURUS 
 (B). The communication 
 with the vestibule is indi- 
 cated by a black spot 
 anteriorly. 
 
LUNGS 
 
 291 
 
 T 
 
 give rise to a tubular and sponge- 
 like meshwork (comp. Fig. 236). 
 
 The lung of Snakes exhibits an in- 
 termediate form, for in spite of the 
 finely-meshed tissue arising from the 
 periphery, it still retains a narrow central 
 cavity. The right lung only is as a rule 
 fully developed in Snakes and Amphis- 
 boenians, owing to the elongated form of 
 the body, while the left remains in a 
 rudimentary condition, or even disap- 
 pears entirely. 
 
 In the Chameleon (Fig. 236) the an- 
 terior portion of the lungs is much more 
 compact and spongy than the posterior, 
 which grows out into numerous sac-like 
 processes, some of which reach as far 
 back as the pelvic region ; their form 
 is very variable, being spindle-shaped, 
 club-shaped, or lobulated, and their 
 walls are very thin ; they extend in 
 amongst the viscera wherever there is 
 room. If these processes have any res- 
 piratory function, it is at most only a 
 very slight one. An indication of a 
 similar arrangement is seen in the lungs 
 of Testudo, in which a single thin- walled 
 process extends backwards to the pelvic 
 region. These processes seem to fore- 
 shadow a condition which reaches its 
 highest development in Birds. 
 
 A uniform ground-plan is to be 
 observed in the arrangement of the 
 intra-pulmonary bronchial system 
 through the whole series of the 
 Amniota, from Crocodiles onwards. 
 A continuation of the bronchus, 
 which is almost straight, always 
 passes through the lung to its pos- 
 terior end. This may be called the 
 main bronchus ; from it a series of 
 lateral bronchi arise. 
 
 Birds. The respiratory appar- 
 atus of Birds presents so many 
 .remarkable peculiarities, both as 
 regards the structure of the lungs 
 
 and in the presence of air-sacs, that it must be considered in some 
 detail. 
 
 The comparatively small but highly vascular lungs (Figs. 237 
 
 and 238) are closely applied to the thoracic vertebrae and heads of 
 
 Uo 
 st 
 
 FIG. 236. LUNGS OF Chamceleo 
 monachus. 
 
 T 7 trachea. 
 
292 COMPARATIVE ANATOMY 
 
 the ribs, and are capable of very little distension. They are pene- 
 trated by a system of bronchi which will be described presently. 
 The lower surface of each lung is closely invested by a thin fibrous 
 membrane, the pulmonary aponeurosis, 1 into which are inserted a 
 variable number of muscular bands (costo-pulmonary muscles) : 
 these arise from the vertebral ribs, and are supplied by the inter- 
 costal nerves (Fig. 238). 
 
 When the ventral body-wall of a Bird is removed, the heart, 
 stomach, liver, and intestine are seen pressed towards the mid-line, 
 and on either side of them a tightly-stretched fascia, the oblique 
 septum, is observable, which shuts them off from a paired lateral 
 sub-pulmonary chamber (Fig. 237). Other chambers are situated 
 in the anterior thoracic region, ventral to the lungs. Others, again, 
 are seen in the region of the heart and in the posterior part of the 
 abdominal cavity. These chambers are occupied by the air-sacs 
 with which certain of the bronchi communicate. 
 
 The most posterior chamber on either side encloses an abdominal 
 (posterior) air-sac (Fig. 237). In Apteryx, this is completely closed in by 
 the oblique septum, but in other Birds it gives rise to a large, distensible 
 diverticulum which extends backwards ventrally to the kidney, amongst the 
 viscera. 
 
 In -front of this there are two air-sacs lying above and externally to the 
 oblique septum, and constituting the main part of the sub-pulmonary 
 chamber ; these may be called the anterior and posterior intermediate sacs. 
 A transverse dividing-wall separates these, at the level of the coeliac 
 artery, and a second septum shuts off the anterior intermediate sac from the 
 one lying in front of it, to be described presently. The posterior inter- 
 mediate air-sac presents the simplest and most constant relations, and never 
 communicates with any of the neighbouring chambers, as is often the case 
 with the anterior intermediate. 
 
 A pair of prebronchial air-sacs lies on either side of the oesophagus above 
 each bronchus, anterior to the hilum of the lung, and below this a sub-bronchial 
 sac is situated, which is separated behind from the anterior intermediate sac 
 by a septum. This is usually unpaired, the sac of either side fusing with its 
 fellow to form an inter clavicular chamber, bounded by the furcula 2 ; it com- 
 municates with neighbouring air-cavities which lie between the pericardium 
 and sternum and in the axilla, outside the body-cavity (axillary sac). 
 
 The main bronchus (mesobronchium) runs close to the ventral surface of 
 the lung surrounded by the lung-parenchyma, and extends to its posterior 
 end, where, as a rule, it opens directly into the abdominal air-sac (Fig. 238). 
 From it a large lateral bronchus is given off, which opens into the posterior 
 intermediate sac by one or two (e.g., in Passeres) apertures. Besides this 
 there are from four to six other lateral bronchi, all of which become 
 broadened out in a fan-like manner on the ventral surface of the lung. 
 These may be called entobronchia (bronchi divergences) : they all arise from 
 the anterior portion of the mesobronchium. The first of these radiates out 
 
 1 The pulmonary aponeurosis, as well as the oblique septum, is often spoken of 
 as a "diaphragm" (comp. p. 141). The chamber (pleural cavity) in which the 
 lungs are situated is shut off from the rest of the abdominal cavity in Chelonians 
 and Crocodiles also. 
 
 2 In some Birds (e.g., Rhea, Vulture, Adjutant) a median septum is present 
 separating the two sub-bronchial sacs. 
 
FIG. 237. ABDOMINAL VISCERA AND AIR-SACS OF A DUCK AFTER THE RE- 
 MOVAL OF THE VENTRAL BODY- WALL. (From a drawing by H. Strasser.) 
 
 T, trachea ; H, heart, enclosed within the pericardium ; rL,lL, right and left 
 lobes of liver ; Ish, suspensory (falciform) ligament, and led, Ics, right and 
 left coronary ligament of the liver ; D, intestine ; P, pectoralis major ; pa, 
 pv, pectoral artery and vein ; S, subclavius muscle ; Cd, coracoid ; F, furcula ; 
 Ifcd, coraco-furcular ligament; Lg, Lg l , lung; r.abd.S, Labd.S, right and 
 left abdominal (posterior) air-sac ; D.th.a, oblique septum ; ft, posterior 
 intermediate air sac ; t, anterior intermediate air- sac ; s 1 , s 1 , partition -walls 
 between these sacs ; s, s, partition walls between the anterior intermediate 
 air-sacs and the unpaired sub-bronchial sac, lying in the anterior part of the 
 body-cavity; v, portion of anterior wall of latter; p, axillary sac lying 
 between the coracoid, scapula, and the anterior ribs, and communicating with 
 the sub-bronchial air- sac ; C, C, prebronchial sacs ; *, point of entrance of 
 the bronchi into the lung ; Ap, pulmonary artery ; Aa, Va, innominate artery 
 and vein with their branches. 
 
294 
 
 COMPARATIVE ANATOMY 
 .--m.l.c 
 
 FIG. 238. LEFT LUNG or THE DUCK, in situ. (From a drawing by H. Strasser. 
 The main bronchus is cut open ; internally to it lies the pulmonary vein, and 
 externally the pulmonary artery. 
 
 Oe, oesophagus; m.l.c, muse, longus colli ; Br. Ws, thoracic vertebrae ; v, v, ends 
 of free vertebral ribs ; #tv, stv, sections of ribs which are connected with the 
 sternum ; N, kidney ; Tr, trachea, /, first entobronchium, and c, its ostium 
 communicating with the prebronchial air-sac ; i, a, e, its internal, anterior, 
 and external branches ; Ili, lie, internal and external branch of the second 
 entobronchium : the end of lie opens into the sub-bronchial sac at d ; ///, 
 third entobronchium, with the aperture for the anterior intermediate air- sac ; 
 IV, fourth entobronchium ; au, opening of the main bronchus into the 
 abdominal sac ; b, opening of the outer lateral branch of the mesobronchium 
 into the posterior intermediate air-sac ; ft 1 , second ostium of the latter, more 
 towards the middle line (present in Passeres). The boundary of the pul- 
 monary aponeurosis is seen along the outer edge of the lung, and the costo- 
 pulmonary muscles are shown extending from it to the ribs. 
 
 anteriorly to the hilum of the lung, and gives off internal, external and 
 anterior branches, one of which opens into the prebronchial sac. The other 
 entobronchia give rise to two series of branches, one of which extends 
 inwards and backwards between the factors of the pulmonary vein, and the 
 other outwards between the arterial branches. Almost without exception 
 a large aperture or ostium is present on the wall on the third entobronchium, 
 
LUNGS 295 
 
 communicating with the anterior intermediate air-sac. A branch of the 
 second entobronchium opens externally to the hilum of the lung into the 
 sub-bronchial sac. 
 
 The lateral bronchi considered as yet have to do with the ventral surface 
 of the lung only ; but besides these there are a variable number of ecto- 
 bronchia arising from the dorsal side of the main bronchus posteriorly to the 
 entobronchia. These come off in a longitudinal row, those of the outer row 
 being larger than those of the inner. They pass dorsally to the costal face 
 of the lung. Both ecto- and entobronchia give off numerous bronchi of a 
 third order, or parabronchia : the walls of these are raised into numerous 
 transverse net-like folds, into which the pulmonary capillaries extend. 
 
 The air-sacs arise from the embryonic pulmonary vesicles as 
 delicate-walled hollow processes, lined by pavement epithelium: these 
 grow rapidly, and soon exceed the lung proper in size, extending 
 amongst the viscera. Their form and extent depend largely upon 
 their surroundings : they consist simply of interstitial cavities lined 
 by the membrane of the air-sacs. Moreover, they are not confined 
 to the body-cavity, but in numerous places extend beyond it, pass- 
 ing in between the muscles, beneath the skin, and even into most 
 of the bones. The latter are thus rendered pneumatic, and con- 
 sequently the specific gravity of the body is lessened, and the power 
 of flight increased. The pneumaticity of the bones is not, however, 
 an essential peculiarity connected with flight, for in many Birds 
 which are extremely good fliers (e.g., Larus, Sterna) the bones are 
 not pneumatic. 1 In these cases, however, a compensation is 
 effected by a more marked development of the muscles, and the 
 abdominal (posterior) air-sac, which in no Birds appears to be 
 entirely wanting, is here well developed. In the cursorial Ratitse, 
 on the other hand, the bones are markedly pneumatic. 
 
 The air-sacs must be looked upon as integral parts of the 
 respiratory apparatus : a greater amount of air can, by their means, 
 pass in and out during inspiration and expiration, especially through 
 the larger bronchi, and consequently there is less necessity for the 
 expansion of the lung-parenchyma. The function of the prolonga- 
 tions of the air-sacs lying towards the outer surface of the body 
 consists in the giving off of watery vapour and in regulating the 
 heat of the body. Those which extend in between the muscles, 
 and supplant the connective and fatty tissue in these regions, have 
 a further importance in causing less power to be lost in friction. 
 
 But by far the greatest importance of the air-sacs situated towards 
 the periphery consists in the enlargement of the anterior thoracic 
 region, principally that surrounded by the pectoral arch. A larger 
 development of the skeleton can thus take place, giving an increase 
 
 1 The pneumaticity of the bones is not a special peculiarity of Birds : amongs 
 Mammals, frontal, maxillary, and sphenoidal sinuses are present in Anthropoids 
 Elephants, and Marsupials for instance ; the skull of Crocodiles is also strongly 
 pneumatic. All these sinuses communicate with one another, and also with the 
 tympanic cavity. They are in many cases developed in order to give a greater 
 surface for the attachment of muscles, and also to effect a saving of material and 
 a lightening of the skull. 
 
296 
 
 COMPARATIVE ANATOMY 
 
 of surface for muscular attachment without any considerable in- 
 crease in weight. Everything, in fact, combines to establish an 
 organ of flight with a large wing-surface and an increased strength 
 of the muscles. 
 
 Mammals. As in Reptiles, the blood-vessels are of funda- 
 mental importance in determining the structure of the bronchial 
 system. The pulmonary artery crosses the main bronchus formed 
 
 by the bifurcation of the trachea at 
 its anterior end, and this point may 
 be taken as dividing the lateral 
 bronchi into two systems an an- 
 terior eparterial and a posterior Jiy- 
 parterial. 
 
 The hyparterial series is always 
 well developed, and consists of a 
 double row of lateral bronchi, be- 
 tween the roots of which the pul- 
 monary artery passes backwards 
 dorsally, while the corresponding vein 
 runs along the median side of the 
 main bronchus (Fig. 239). The epar- 
 terial system, on the other hand, 
 gradually becomes of much less im- 
 portance and in certain cases is re- 
 presented only by a single external 
 lateral bronchus on either side (Fig. 
 239) ; and, as a rule, even the left of 
 these disappears, only the right re- 
 
 fl|P: JMJJ\ maining, and even this is not always 
 
 retained. The eparterial bronchus, 
 whether developed on one or on both 
 
 instead of from the main bronchus. 
 
 In by far the greater number of 
 Mammals, then, the left eparterial 
 bronchus has disappeared, while the 
 
 FIG. 239. DIAGRAM OF THE AR- 
 RANGEMENT OF THE BRONCHI 
 IN MAMMALS. (From the ven- 
 tral side.) 
 
 a, a, eparterial bronchus of either 
 side ; b, series of ventral, and 
 
 , 
 monary ven the anterior lobe of the right lung 
 
 belongs to the eparterial and 1hat of 
 
 the left lung to the first hyparterial bronchus, these lobes are 
 evidently not homologous, the middle right lobe corresponding 
 much more nearly to the anterior lobe of the left side. There is 
 thus a want of symmetry between the right and left sides, the 
 right lung usually retaining one more element than the left 
 (Fig. 240A). The so-called accessory fourth lobe does not correspond 
 to a true lobe, but represents the main axis of the lung enclosing 
 the main bronchus. 
 
LUNGS 
 
 297 
 
 The cartilages of the bronchi become more and more sparse and 
 finally disappear as the latter divide up into finer and finer 
 branches. 
 
 The thoracic cavity is lined by a serous membrane, the pleura, 
 in which, as in the case of the peritoneum (p. 235), a parietal and 
 
 T* 
 
 FIG. 240A. LUNG OF MAN. (From the ventral side. ) 
 
 1, 2, 3, lobes of the right, and 2a, 3a, of the left lung ; Z, base of lung ; t, incisura 
 cordis ; S, sulcus for the subclavian artery ; Tr, trachea. 
 
 FIG. 240s and c. DIAGRAM OF THE PLEURAL AND PERICARDIAL CAVITIES OF 
 MAMMALS, FOUNDED ON THE RELATIONS OF THESE PARTS IN MAN. (B, 
 
 horizontal section ; C, transverse section. ) 
 
 Tr, trachea ; Br, bronchi ; L, L, lungs ; H, heart ; W, vertebral column ; P, 
 parietal, and P 1 , visceral layer of the pleura ; ft, points at which these pass 
 into one another at the hilum pulmonalis (Hi) ; m, mediastinum ; PC, Ps l , 
 parietal and visceral layers of the pericardium ; If, ribs (wall of thorax) ; S, 
 sternum. 
 
 visceral layer may be distinguished (Fig. 240, B, C) : the latter is 
 spoken of as the pulmonary pleura, the former as the costal pleura. 
 
298 COMPARATIVE ANATOMY 
 
 Towards the middle line, the pulmonary pleura of either side is 
 reflected so as to form a septum between the right and left thoracic 
 cavities. This septum is called the mediastinum, and the space 
 between its two layers the mediastinal space : through this, the 
 aorta, oesophagus, and postcaval vein run, and in the region of 
 the heart the mediastinum is reflected over the parietal layer of 
 the pericardium. 
 
 There is a lymphatic fluid between the two layers of the pleura 
 which renders the movements of the lungs smooth and easy. 
 
 ABDOMINAL PORES. 
 
 By the term abdominal pore is understood a perforation usually 
 paired of the posterior end of the wall of the peritoneal cavity 
 which puts the coelome into direct communication with the 
 exterior. 1 
 
 In Cyclostomes a pair of pores opens into the urinogenital sinus, 
 serving to conduct the generative products to the exterior : they 
 probably do not correspond to the abdominal pores of other forms, 
 which never have this function, and are better described as genital 
 pores. 
 
 In the Holocephali and Elasmobranchii the abdominal pores are 
 usually paired and are situated posteriorly to the cloaca (Figs. 206, 
 289, 290), and may be enclosed within its lips. They are wanting in 
 the NotidanidaB, Cestracionidse, and Rhinidse, and are not con- 
 stantly present in the Scylliidse, even in individuals of the same 
 species. In Ganoids, they open between the urinogenital aperture 
 and anus, but are apparently wanting in Amia. Amongst Teleosts, 
 they are said to be present only in the SalmonidaB and Mormyridse, 
 right and left of the anus ; but even in these, the pore of one or 
 of both sides may be absent. In the Mursenidse, there is a single 
 genital pore, which is apparently more nearly comparable to the 
 similarly named structure in other Teleosts (see under Generative 
 Organs) and to the genital pores of Cyclostomes. In Ceratodus the 
 abdominal pores are paired, and open behind the cloaca, while in 
 Protopterus a single, apparently blind, canal is present on the same 
 side of the ventral fin as the vent (Fig. 209), sometimes to the 
 right and sometimes to the left of the middle line, either within or 
 without the sphincter of the cloaca. 
 
 Abdominal pores are not known to occur in the Amphibia and 
 Mammalia, but amongst Reptiles they are perhaps represented by the 
 peritoneal canals of the Chelonia and Crocodilia, which in the 
 former are in close relation with the penis or clitoris, and usually 
 end blindly. 
 
 1 The abdominal pores may possibly correspond to the remains of segmental 
 ducts. Other connections of the crelome with the exterior (by means of the 
 nephrostomes of Anamnia and the ostia of the oviducts in the majority of 
 Vertebrata) will be mentioned in a subsequent chapter. 
 
H. ORGANS OF CIRCULATION. 
 
 (VASCULAR SYSTEM.) 
 
 THE organs of circulation, which arise from the mesoblast, 1 
 consist, in the Craniata, of a hollow central muscular organ, the 
 heart, which is connected with a series of completely closed tubes, 
 the blood-vessels. The heart and blood-vessels contain a coloured 
 fluid, the Uood, and their cavities probably represent the remains 
 of the blastoccele (p. 4). Another system of vessels containing a 
 colourless fluid, the lymph, must be distinguished from the blood 
 vessels : lymph, however, is present in various spaces or sinuses 
 in the body as well as in the lymph -vessels (p. 333) : the lymphatic 
 system is, .therefore, not completely closed, the vessels communicat- 
 ing with the sinuses on the one hand, and with the blood-vessels on 
 the other. 
 
 Both blood and lymph consist of a colourless fluid, the plasma, 
 in which float numerous cells or corpuscles. The blood-corpuscles are 
 of two kinds colourless, nucleated, amoeboid cells, known as white 
 or colourless corpuscles or leucocytes, and far more numerous red 
 Hood- corpuscles or erythrocytes? The lymph contains colourless 
 corpuscles only, and these are precisely similar to those of the 
 blood. Both blood and lymph are kept in constant circulation 
 through the vessels by the contraction of the heart, which acts 
 both as a force-pump and a suction-pump, and they serve to carry 
 the absorbed food and oxygen to, and the waste products from, all 
 parts of the body. 
 
 All the blood vessels which bring back the blood to the heart 
 are known as veins, while those which carry it from the heart 
 are called arteries : the latter usually contain oxygenated, the 
 former impure blood, but this is by no means always the case. 
 Many of the veins are provided with valves, which are adapted to 
 prevent the reflux of the blood : they have the form of semilunar 
 folds of the internal coat, and each is usually made up of two folds, 
 
 1 According to some embryologists the hypoblast also takes part in the forma- 
 tion of the vascular system. 
 
 2 In Amphioxus the blood contains white corpuscles only ; there is no heart, 
 and the vessels are only partially comparable to those of the Craniata. 
 
300 COMPARATIVE ANATOMY 
 
 placed opposite to one another. The arteries (and also certain of 
 the veins) divide up into smaller and smaller branches, eventually 
 giving rise to microscopic tubes called capillaries, the walls 
 of which consist merely of a single layer of epithelial cells, 
 and these again unite to form the factors of the veins. The walls 
 both of veins and arteries consist, in addition to the epithelium, of 
 connective and elastic tissue and of unstriated muscular fibres, and are 
 much thicker in the case of the arteries than in that of the veins, 
 in which the muscular elements may be altogether wanting. 
 
 The nucleus of the red corpuscles persist, and the whole cell is biconvex, 
 in all Vertebrates below Mammals ; and, even in these, nucleated red cells 
 may be seen in the marrow of the bones, in the blood of the spleen, and 
 often in that of the portal vein. In all other parts of the body of Mammals 
 they lose their nuclei and become biconcave. In all Mammals, except the 
 Camelidse, the red corpuscles have the form of circular discs ; in the last- 
 mentioned group and in all other Vertebrates except Cyclostomes they are 
 oval. They are largest in certain Urodeles, being in Amphiuma as much 
 as 75/x in their longest diameter ; then come, in order, those of other 
 Urodeles and of Dipnoans, Reptiles, Anurans, Fishes, Birds, and Mammals, 
 in which latter order they are the smallest, varying in different families from 
 2'5/x (Tragulidre) to 10/*. 
 
 The heart is enclosed within a serous membrane, the pericar- 
 dium (Fig. 240c), which consists of parietal and visceral layers ; 
 the former is invested by the mediastinum (p. 298), and the latter 
 is closely applied to the heart. Between the two layers is a space 
 filled with lymph, representing part of the ccelome ; this is usually 
 completely shut off from the abdominal cavity, but in Elasmo- 
 branchs the two communicate by means of pericardio-peritoneal 
 canals. 
 
 The heart arises either as a single (Elasmobranchii, Amphibia) 
 or as a paired (Teleostei, Sauropsida, Mammalia) tubular cavity 
 in the splanchnic layer of the mesoblast (comp. note on p. 299) 
 along the ventral region of the throat, close behind the gill-clefts. 
 Its wall becomes differentiated into three layers, an outer serous 
 (pericardial), a middle muscular, and an inner epithelial. In this 
 respect it essentially corresponds with the larger vessels, in the walls 
 of which, as already mentioned, three layers can also be distinguished; 
 but in the heart the muscular fibres are striated. By a study of its 
 development we thus see that the heart corresponds essentially to 
 a strongly developed blood-vessel, which at first lies more or less in 
 the longitudinal axis of the body ; later, however, it becomes much 
 more complicated by the formation of various folds and swellings. 
 Thus the embryonic tubular heart becomes folded on itself and 
 divided into two chambers, an atrium or auricle and a ventricle 
 (Fig. 241). Between these, valvular structures arise, which only 
 allow the blood to flow in a definite direction on the contraction of 
 the walls of the heart, viz., from the atrium to the ventricle ; any 
 backward flow is thus prevented. The valves are formed by 
 
VASCULAR SYSTEM 
 
 301 
 
 a differentiation of the muscular trabeculse of the walls of 
 the heart. The atrium, into which the blood enters, represents 
 primitively the venous portion of the heart ; the ventricle, from 
 which the blood flows out, corresponding to the arterial portion. 
 The venous end further becomes differ- 
 entiated to form another chamber, the 
 sinus venosus, and the arterial end gives 
 rise distally to a truncus arteriosus ; the 
 proximal end of this (conns arteriosus) is 
 provided with more or less numerous 
 valves, and its distal end (bulbus arteriosus} 
 is continued forwards into the arterial 
 vessel (ventral aorta). 
 
 The ventral aorta gives off right and 
 left a series of symmetrical afferent bran- 
 chial arteries (Figs. 242, 243), each of 
 which runs between two consecutive gill- 
 clefts, branches out into capillaries in the 
 gills, when present, and then becomes con- 
 tinuous with a corresponding efferent 
 branchial artery. After the first pair of 
 these has given off branches to the head 
 (carotids), they all unite above the clefts 
 to form a longitudinal trunk on either 
 side, and there form the right and left roots 
 of the dorsal aorta ; this extends back- 
 wards along the ventral side of the ver- 
 tebral axis into the tail as a large unpaired trunk, which gives off 
 numerous branches including paired vitelline or omphalo-mesenteric 
 arteries to the yolk-sac, and (except in Fishes and Dipnoans) 
 allantoic arteries to the embryonic urinary bladder or allantois 
 (pp. 9 and 337, and Figs. 8, 9, 242, 244). 
 
 Primarily, the blood becomes purified in the vessels which 
 branch out over the yolk-sac, from whence it is returned by the 
 vitelline or omphalo-mesenteric veins (Fig. 244). These join with 
 the allantoic veins and veins of the alimentary canal to form what 
 eventually becomes the hepatic portal vein, which divides up into 
 capillaries in the liver. These capillaries then unite to form the 
 hepatic veins, which open into the sinus venosus. 
 
 Into the sinus venosus there also opens on either side a pre- 
 caval vein or anterior vena cava, which receives an anterior cardinal 
 or jugular vein from the head, and a posterior cardinal vein from 
 the body generally (not including the alimentary canal). The 
 caudal vein which lies directly below the caudal aorta, is con- 
 nected with the posterior cardinals, usually indirectly, through the 
 renal portal veins (comp. Fig. 264). The further development of 
 the embryonic vessels may take place in one of three ways. 
 
 The embryo may either leave the egg, and take on an aquatic 
 
 A 
 
 Sw 
 
 FIG. 241. DIAGRAM SHOW- 
 ING THE PRIMITIVE RE- 
 LATIONS OF THE DIFFER- 
 ENT CHAMBERS OF THE 
 HEART. 
 
 Sv, sinus venosus, into 
 which the veins from the 
 body open ; A, atrium ; 
 V, ventricle ; Ca, conus 
 arteriosus ; Ba, bulbus 
 arteriosus, from which the 
 main artery arises. 
 
302 
 
 COMPARATIVE ANATOMY 
 
 existence (Anamnia), making use of its branchial vessels for par- 
 poses of respiration, the entire allantois, in the case of the Am- 
 
 FIG. 242. DIAGRAM or THE EMBRYONIC VASCULAR SYSTEM. 
 (The portal systems are not shown. ) 
 
 A, A, dorsal aorta ; RA, RA, right and left roots of the aorta, which arise from 
 the branchial vessels, Ab, by means of the collecting trunks, S, S 1 ; c, c 1 , 
 the carotids; Sb, subclavian artery; KL, gill-clefts ; Si, sinus venosus ; A, 
 atrium ; V, ventricle ; B, truncus arteriosus ; Vm, vitelline veins ; Am, 
 vitelline arteries ; Ic, Ic, common iliac arteries ; E, E, external iliac arteries ; 
 All, allantoic (hypogastric) arteries; Acd, caudal artery; VC, HC, anterior 
 and posterior cardinal veins ; Sb 1 , subclavian vein ; D, precaval veins (ductus 
 Cuvierii), into which the anterior and posterior cardinals open. 
 
 phibia, giving rise to the bladder. In the Amniota, which from 
 the first breathe by means of lungs, a modification and reduction of 
 
VASCULAR SYSTEM 
 B 
 
 303 
 
 ca. 
 
 D E F 
 
 FIG. 243. DIAGRAM OF THE ARTERIAL ARCHES OF VARIOUS VERTEBRATES. 
 
 (After Boas.) 
 
 A, embryonic condition ; B, Fish ; <7, Urodele ; Z>, Reptile (Lizard) ; E, Bird ; 
 F, Mammal. The parts which disappear are dotted. 
 
 k and h, the two first embryonic arches, which almost always disappear ; 1 4, 
 the four more posterior arches ; I 1 and 3 1 , first and third afferent branchial 
 arteries ; 1" and 3", the corresponding efferent branchial arteries ; 2 in D and 
 F, second arch of the left side ; 2 1 in D, E and F, second arch of the right 
 side ; a, b, c, the vessels into which the ventral arterial trunk is divided in 
 Reptiles, Birds, and Mammals ; ao, dorsal aorta ; ca, carotid ; I, pulmonary 
 artery ; 8 (in F}, left subclavian artery ; st, and s (in B), ventral aorta. 
 
304 
 
 COMPARATIVE ANATOMY 
 
 the branchial vessels and allantois takes place, and the latter may 
 even disappear entirely (see under Urinary organs). In the third 
 case, the embryo undergoes a longer intra-uterine existence, the 
 allantois coming into close connection with the walls of the uterus 
 by means of the chorionic villi : the allantoic vessels extend into the 
 wall of the uterus and come into more or less close relations with 
 
 SJT. 
 
 Jt.0f. 
 
 V.Ca, 
 
 JlQf.S 
 
 I,.of.A 
 
 FIG. 244. DIAGRAM OF THE CIRCULATION or THE YOLK-SAC AT THE END or 
 THE THIRD DAY OF INCUBATION IN THE CHICK. (After Balfour.) 
 
 H, heart ; A A, the second, third, and fourth aortic arches : the first has become 
 obliterated in its median portion, but is continued at its proximal end as the 
 external carotid, and at its distal end as the internal carotid ; Ao, dorsal 
 aorta; L. Of. A, left vitelline artery; P. Of. A, right vitelline artery; S. T, 
 sinus terminalis ; L. Of, left vitelline vein ; R.Of, right vitelline vein; S. V, 
 sinus venosus ; D.C, ductus Cuvieri ; S.Ca.V, anterior cardinal or jugular 
 vein; V.Ca, posterior cardinal vein. The veins are marked in outline, and 
 the arteries are made black. The whole blastoderm has been removed from 
 the egg, and is supposed to be viewed from below. Hence the left is seen on 
 the right, and vice versa. 
 
 the maternal vessels, thus serving for the respiration and nutrition 
 of the fcetus. In this way a placenta and a placental circulation 
 arise (comp. Fig. 9, and pp. 9 and 337). 
 
 On the appearance of pulmonary respiration, important changes 
 take place in the branchial vessels and heart. The formation 
 of a septum in both the atrium and ventricle leads to the presence 
 
VASCULAR SYSTEM 305 
 
 of two atria or auricles, and two ventricles, and the conus arteriosus 
 and sinus venosus become eventually more or less incorporated in 
 the ventricles and right auricle respectively. Thus a right (venous) 
 and a left (arterial) half can be distinguished ; and a new vessel, 
 the pulmonary artery, arising from the last arterial arch, becomes 
 connected with the right ventricle : this conveys venous blood to the 
 lungs, while special vessel $ (pulmonary wins} re turn the oxygenated 
 blood from the lungs to the left auricle, from which it passes into the 
 left ventricle and so into the general circulation of the body. 
 
 The branchial vessels never become functional as such, in any 
 period of development either in Sauropsida or Mammalia ; but those 
 which persist give rise, as already mentioned, to important vascular 
 trunks of the head, neck (carotids), anterior extremities (sub- 
 clavians), and lungs (pulmonary arteries), and also to the roots of 
 the aorta, one or both of which may remain (comp. Fig. 243). 
 
 The primitive number of arterial arches is six, the first two of 
 which (belonging to the mandibular and hyoid arches respectively) 
 almost always disappear early : in caducibranchiate Amphibia 
 (including Anura) and in Amniota, the fifth arch also disappears. 
 The third gives rise to the carotid arch ; the fourth of both sides 
 (Amphibia, Reptilia), or of one side (Aves, Mammalia), to the aortic 
 or systemic arch, and the sixth to the pulmonary arch (Fig. 243). 
 
 From the Dipnoi onwards, the posterior cardinals become more 
 or less completely replaced functionally by a large unpaired vein, 
 ihepostcaval or posterior vena cam, which opens independently into 
 the right auricle. 
 
 THE HEART, TOGETHER WITH THE ORIGINS OF THE MAIN 
 
 VESSELS. 
 
 Fishes (including Cyclostomes). The heart in Fishes is 
 situated in the anterior part of the body-cavity, close behind the 
 head. It is formed on the same general plan as that described on 
 p. 300, consisting of a ventricle with a truncus arteriosus or 
 merely a bulbus (Cyclostomi, Teleostei), and an atrium or 
 auricle, the latter receiving its blood from a sinus venosus, and 
 being laterally expanded to form the appendices auricula (Figs. 245 
 and 246). 
 
 In correspondence with the work which each portion has to 
 perform, the walls of the atrium are thin, while those of the ventricle 
 are much stronger, its muscles giving rise in the interior to a network 
 and usually to a series of large trabeculae : this holds good through- 
 out the Craniata. 
 
 Between the sinus venosus and atrium, and also between 
 ventricle and atrium, membranous valves are present ; there are 
 primarily two atrioventricular valves, but they may be further sub- 
 divided. Numerous valves, arranged in rows, are present in the 
 
 x 
 
306 
 
 COMPARATIVE ANATOMY 
 
 muscular conus arteriosus (Fig. 246, A) : these are most numerous 
 in Elasmobranchs and Ganoids. There is a tendency, however, for 
 the posterior valves, or those which lie nearest the ventricle, 
 
 D.C.s 
 
 FIG. 245. HEART OF A, Zygcma malleus, FROM THE VENTRAL SIDE ; B, OF Acan- 
 thias vulyaris, FROM THE DORSAL SIDE, WITH THE ATRIUM CUT OPEN (after 
 Rose) ; C, OF A TELEOST (Silurus glanis). 
 
 A, A, atria ; a, a, auricular appendages ; F, ventricle ; tr (in B) and Ba (in C), 
 
 bulbus arteriosus ; tr (in A) and co (in B), conus arteriosus, tr (in C), ventral 
 
 aorta. 
 D.C.d and D.C.s, right and left precavals ; V.a.d. and V.a.s, right and left 
 
 valve of the sinus venosus ; O.a.v, atrio- ventricular aperture ; l,a 4,a, 
 
 afferent branchial arteries. 
 
 gradually to undergo reduction (B). The most anterior row always 
 persists, and corresponds to the single row of valves between the 
 ventricle and bulbus in Teleosts (c) and Cyclostomes. Together 
 with the reduction of these valves, the conus arteriosus also be- 
 comes reduced in the last-mentioned forms, so that the non- 
 
VASCULAR SYSTEM 
 
 30' 
 
 contractile bulbus arteriosus usually lies close against the ventricle 
 (Fig. 246, c). 
 
 The heart of Fishes contains venous blood only, which it forces 
 
 C 
 
 FIG. 246. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE HEARTS OF 
 VARIOUS FISHES. (From Boas's Zoology.) A, Fish with well developed 
 conus anteriosus (e.g., Elasmobranch) ; B, Amia ; C, a Teleost. In B and C 
 the sinus venosus and atrium are not indicated. 
 
 a, atrium ; 6, bulbus arteriosus ; c, conus arteriosus ; k, valves ; s, sinus venosus ; 
 t, ventral aorta ; v, ventricle. 
 
 through the afferent branchial arteries (Figs. 243, B, 245, c, and 264) 
 into the capillaries of the gills, where it becomes oxygenated, to 
 pass thence into the efferent branchial arteries, and so into the 
 aortic roots. 
 
 Dipnoi. In the Dipnoi, as in Fishes proper, the heart lies 
 far forwards, near the head. In correspondence with the double 
 mode of respiration, by lungs as well as by gills, it reaches a 
 stage of development mid- way between that seen in Elasmo- 
 branchs and in Amphibians (Figs. 247 and 248). The atrium 
 becomes divided into a left and a right chamber by a septum, as 
 does also the ventricle to some extent, owing to the presence of 
 a cushion composed of muscular fibres and connective-tissue (Fig. 
 247) situated between the atrium and ventricle, and extending 
 into both of these chambers : this acts as a valve, ordinary atrio- 
 ventricular valves being absent. The sinus venosus, from the 
 Dipnoi onwards, opens into the right atrium. 
 
 The conus arteriosus is twisted spirally on itself (Fig. 248) : in 
 Ceratodus it is provided with eight transverse rows of valves, and 
 
 x 2 
 
308 
 
 COMPARATIVE ANATOMY 
 r/3osl.car 
 
 W 
 
 FIG. 247. 
 
 FIG. 248. 
 
 FIG. 247. HEART OF Protopterus annecten*. From the left side, part of the 
 wall of the left atrium being removed. (After Rose.) 
 
 W, fibrous cushion extending into the ventricle ; Si.v, sinus venosus, within 
 which the pulmonary vein (Lv) extends to open into the left auricle by a 
 valvular aperture ; L. Vh and E. Vh, left and right atria ; S.a, septum 
 atriorum ; Co, conus arteriosus. 
 
 FIG. 248. Ceratodus forsteri. DIAGRAMMATIC VIEW OF THE HEART AND MAIX 
 BLOOD VESSELS AS SEEN FROM THE VENTRAL SURFACE. (From Parker and 
 Haswell's Zoology, after Baldwin Spencer. ) 
 
 aff. I, 2, 3, 4, afferent branchial arteries ; I br, 2 br, 3 br, 4 br, position of gills ; 
 c.a, conus arteriosus; d.a, dorsal aorta; d.c, ductus Cuvieri ; epi. I, epi.2, 
 epi.3, epi. 4, efferent branchial arteries ; hy.art, hyoidean artery ; i. v. c, post- 
 caval vein ; l.ant.car, left anterior carotid artery; Laur, left auricle; Lbr.v, 
 left brachial vein; l.juy.v, left jugular vein ; I. post. car, left posterior caro- 
 tid artery ; I. post. card, left posteri or cardinal vein ; l.pul.art, left pulmonary 
 artery ; Lsc.v, left sub-scapular vein ; r. ant. car, right anterior carotid artery ; 
 r.aur, right auricle ; r.br.v, right brachial vein ; r.jug.v, right jugular vein ; 
 r. post. car, right posterior carotid ; r.puLart, right pulmonary artery ; r.sc.v, 
 right sub- scapular vein ; vent, ventricle. 
 
 begins to be divided into two chambers. In Protopterus this divi- 
 sion is complete, so that two currents of blood, mainly arterial and 
 
VASCULAR SYSTEM 309 
 
 mainly venous respectively, pass out from the heart side by side. 
 The former comes from the pulmonary vein, from which it passes 
 into the left atrium, thence into the left side of the ventricle, and 
 so to the two anterior branchial arteries. The venous current, 
 on the other hand, passes from the right side of the ventricle into 
 the third and fourth afferent branchial arteries and thence to the 
 corresponding gills, where it becomes purified ; it reaches the aortic 
 roots by means of the efferent branchial arteries. The paired pul- 
 monary artery arises from the fourth efferent branchial in Ceratodus 
 (Fig. 248), and from the aortic root in Protopterus and Lepidosiren, 
 that of the right side bifurcating to supply the dorsal surface of the 
 lung or lungs (p. 288), while that of the left side supplies the 
 ventral surface. The two pulmonary veins unite to form a median 
 trunk which becomes closely connected with the sinus venosus, so 
 as to appear sunk within its walls (Fig. 247). Thus the blood 
 becomes once more purified before it passes into the left ventricle. 
 Apostcaval vein, present from the Dipnoi onwards, opens into the 
 sinus venosus posteriorly to the precavals, and with it the hepatic 
 veins communicate (Figs. 248 and 267). 
 
 Amphibia. With the exception of the Gymnophiona, in which 
 it is situated some distance back, the heart in all Amphibians lies far 
 forwards, below the anterior vertebrae. A septum atriorum is well 
 developed, but in Urodela and Gymnophiona it is more or less fenes- 
 
 Cl-CP, 
 
 v 
 
 FIG. 249. DIAGRAM SHOWING THE COCRSE OF THE BLOOD THROUGH THE HEART 
 IN Urodela (A) AND Anura (B). 
 
 A, right atrium ; A\ left atrium ; F, ventricle; tr, conus arteriosus, divided -in 
 Anura (B) into two portions, tr, tr* : .through tr venous blood passes into the 
 pulmonary arteries, Ap 1 , Ap l , while through tr 1 mixed blood goes to the 
 carotids, ci ce, and to the roots of the aorta, RA ; fr, lr, pulmonary veins ; 
 v, v, pre- and post-cavals (only one precaval is indicated). ..- 
 
 trated (Fig. 250). There are always two fibrous, pocket-like atrio- 
 ventricular valves, which are connected with the walls of the 
 ventricle by cords. The two pulmonary veins unite before opening 
 into the left atrium. 
 
310 
 
 COMPARATIVE ANATOMY 
 
 The cavity of the ventricle is unpaired, and neither in Urodela 
 nor Anura shows any trace of a septum, so that the blood passing 
 out from it must have a mixed character (Fig. 249). The ven- 
 tricle is usually short and compressed, but is more elongated in 
 Amphiuma, Proteus, and the Gymnophiona. It is continued an- 
 teriorly into a conns arteriosus, as in Elasrnobranchs, Ganoids, and 
 Dipnoans ; this has usually a slight spiral twist, and possesses a 
 transverse row of valves at either end, as well as a spiral fold ex- 
 tending into its lumen. 1 This holds good for the Axolotl, Amblystoma, 
 
 Vcd. 
 
 FIG. 250. HEART OF Cryptobrauchu* japonicus. From the ventral side. (After 
 Rose.) Ta3 left atrium is cut open. 
 
 S.a, septum atriorum, perforated by numerous small apertures ; L.v, L.v 1 , the 
 two pulmonary veins, opening by a single aperture into the left atrium ; O.av, 
 atrio- ventricular aperture ; l a , 4 a , the four arterial arches ; P.d. andP.s, left 
 and right pulmonary arteries ; tr, truncus arteriosus ; L. Vh, K. Vh, left and 
 right atria ; V.s.d and F.*., subclavian veins ; V.j.d and V.j.s, jugular veins ; 
 V.c.d, F.c.-s, posterior cardinal veins ; V.c.i, postcaval vein. 
 
 Salamandra, Amphiuma, and Siren. In others (e.g., Necturus, 
 Prol.eus, Gymnophiona), retrogression is seen in a lengthening of 
 the conus, the disappearance of the spiral fold, and the presence of 
 only a single row of valves. 
 
 'in Ariura, the fold lying within the conus extends so far back 
 that no undivided portion of the cavity is left. The consequence 
 of this is that the blood passing into the hindermost pair of the 
 arterial arches that from which the pulmonary arteries arise is 
 mainly venous, while the others contain more or less mixed blood 
 (Fig. 249, B) ; for, owing to the spongy nature of the ventricle, there 
 1 This spiral fold corresponds to a series of fused valves. 
 
VASCULAR SYSTEM 311 
 
 is no time for its contained blood to get thoroughly mixed before it 
 is forced into the conus. 
 
 As in the Dipnoi, four afferent branchial arteries (Fig. 250) 
 arise on either side from the short conus in the Amphibia, which 
 taking as a type the larva of Salamandra have the follow- 
 ing relations (comp. Fig. 243, c). 
 
 The three anterior arteries pass to numerous external gill-tufts, 
 in which they break up into capillaries (Fig. 251). From the latter 
 three efferent vessels arise, which pass to the dorsal side, and there 
 unite on either side to form the aortic root. The fourth afferent 
 
 FIG. 251. THE ARTERIAL ARCHES OF THE LARVA OF A SALAMANDER. (Slightly 
 diagrammatic.) (After J. E. V. Boas.) 
 
 tr, truncus arteriosus ; 1 to 3, the three afferent branchial arteries ; / to ///, the 
 corresponding efferent arteries ; 4, the fourth arterial arch, which becomes 
 connected with the pulmonary artery (Ap) ; a, a, direct anastomoses between 
 the second and third afferent and efferent branchial arteries ; re, external 
 carotid ; ci, internal carotid ; f, net-like anastomoses between the external 
 carotid and the first afferent branchial artery, which give rise later to the 
 " carotid gland" ; RA, aortic roots ; Ao, dorsal aorta. The arrows show the 
 course which the blood takes. 
 
 branchial artery, which is smaller than the others, does not pass to 
 a gill, but to the pulmonary artery, which arises from the third 
 efferent branchial. The pulmonary artery, therefore, contains far 
 more arterial than venous blood, and thus the lungs of the Sala- 
 mander larva, like the air-bladder of Fishes, can only be of 
 secondary importance in respiration. 
 
 The internal carotid arises from the first afferent branchial 
 artery, towards the middle line, the external carotid coming off 
 further outwards (Fig. 251). The latter, as it passes forwards, 
 becomes connected with the first afferent branchial by net-like anas- 
 tomoses, and these give rise later to the so-called " carotid gland" l 
 
 1 The " carotid gland " loses its character as a refe mirabile (comp. p. 333), 
 and in the adult consists simply of a muscular vesicle with septa in its interior. 
 
312 COMPARATIVE ANATOMY 
 
 of the adult, which probably functions as an accessory heart. Direct 
 connections exist between the second and third afferent and 
 efferent arteries. 
 
 Towards the end of the larval period, the second efferent bran- 
 chial artery increases considerably in relative size, and the fourth 
 ' arterial arch also becomes larger. By a reduction of the anasto- 
 mosis with the' third arch, the fourth carries most of the blood for 
 the pulmonary artery, and the latter thus now contains more venous 
 than arterial blood. When branchial respiration ceases, the anasto- 
 moses between the afferent and etferent branchial arteries no longer 
 consist of capillaries, but a direct connection between them be- 
 comes established (Fig. 252). Finally, the connection between the 
 
 FIG. 252. ARTERIAL ARCHES OF AN ADULT Salamandra maculosa, SHOWN 
 SPREAD OUT. (After J. E. V. Boas. ) 
 
 co, tr, truncus arteriosus ; 1 to 4, the four arterial arches ; ce, external carotid ; 
 cd, " carotid gland " ; ci, internal carotid. The fourth arterial arch, which gives 
 rise to the pulmonary artery (Ap), has increased considerably in size relatively, 
 and is only connected by a delicate ductus Botalli (f) with the second* and 
 third arches ; RA , root of the aorta ; ce, cesophageal vessels. 
 
 first and second branchial arches disappears, the former giving 
 rise to the carotid and the latter forming the large aortic root ; 
 an anastomosis remains throughout' life, however, between the 
 fourth arch, which forms the pulmonary artery, and the second and 
 third arches. This is usually spoken of as the ductus Botalli. 
 
 The third arch varies greatly in its development ; it may be 
 present on one side only, or may even be entirely wanting. 
 
 In the larva) of Anura there are also four afferent branchial 
 arteries present on either side, but these are connected with the 
 corresponding efferent vessels by capillaries only, there being no 
 direct anastomoses (compare Fig. 251). The consequence of this 
 is that all the blood becomes oxygenated. 
 
 In the adult Frog the third arterial arch becomes entirely 
 
VASCULAR SYSTEM 
 
 313 
 
 obliterated, and there is no ductus Botalli : the other vessels re- 
 semble those of the Salamander. In lungless forms (p. 290) a 
 correlative reduction of the pulmonary vessels occurs. 
 
 Reptiles. As in all Amniota, the heart of Reptiles arises far 
 forwards in the neighbourhood of the gill-clefts, but on the forma- 
 tion of a neck it comes to lie much further back than is the case in 
 
 FIG. 253. HEART OF A, Lacerta muralis, AND B, 
 OF A LARGE Varanus, SHOWN CUT OPEN ; C, 
 DIAGRAM OF THE REPTILIAN HEART. 
 
 V, V 1 , ventricles ; A, A\ atria ; tr, Trca, innomi- 
 nate trunk ; 1, 2, first and second arterial 
 
 HA W **"** arches ; Ap, Ap l , pulmonary arteries ; Vp, 
 
 pulmonary vein ; t and *, right and left 
 
 aortic arches ; RA, root of aorta ; Ao, dorsal aorta ; Ca, Ca l , carotids ; Asc, A$, 
 subclavian arteries. /, jugular vein ; V#> subclavian vein ; Ci, postcayal : these 
 three veins open into the sinus venosus, which lies on the dorsal side of the 
 heart, above the point indicated by the letter S. In the diagram C the pre- and 
 postcavals are indicated by Ve, Ve, only one precaval being represented. 
 
 the Anamnia. 1 The carotid arteries and jugular veins are thus 
 correspondingly elongated. 
 
 The principal advance in structure as compared with the Am- 
 phibian heart is seen in the appearance of a muscular ventricular 
 
 1 It is situated furthest forwards in the majority of Lizards and in Chelonians : 
 in Amphisbienians, Snakes and Crocodiles it lies much further back. 
 
314 
 
 COMPARATIVE ANATOMY 
 
 septum, which may be incomplete, as in Lizards (Fig. 253, B), 
 Snakes, and Chelonians, or complete, as in Crocodiles. 
 
 The conns arteriosus now becomes practically absorbed into 
 the ventricular portion of the heart, and each aortic root may be 
 made up at its origin of two arches, anastomosing with one another 
 (Lacerta, Fig. 243, A), or of one only (certain Lizards, Snakes, 
 Chelonians, and Crocodiles, Figs. 253, B, 255), from which the 
 carotid artery arises directly. The left and right aortic arches cross 
 at their base, so that the left arises on the right side, and vice 
 versa. 1 The most posterior arterial arch gives rise to the pul- 
 monary artery (comp. Fig. 243, D). 
 
 The blood from the right ventricle passes into the pulmonary 
 artery as well as into the left aortic arch, and, according as the septum 
 
 A n.s- 
 
 L.v 
 
 Ao.aU. 
 
 FIG. -254. HEART OF Cydodus boddaertei. From the dorsal side. (After Rose). 
 The sinus venosus is almost entirely absorbed into the right atrium. 
 
 D.CU, D.C.d, precaval veins; V.c.i, postcaval vein; V'.j.d, jugular, V.s.d, sub- 
 clavian, and V.G.d. posterior cardinal vein of the right side. L.v, pulmonary 
 vein ; P.s, P.d, pulmonary arteries ; An.*, An, innominate arteries ; Ao.aM, 
 dorsal aorta ; Sp.i, spatium intersepto-valvulare (comp. Fig. 257). 
 
 ventriculorum is complete or incomplete, is either entirely venous 
 (Crocodiles) or mixed (other Reptiles, Fig. 253, c). 
 
 The valves of the heart have undergone a considerable reduction 
 in Reptiles : at the origin both of the aorta and of the pulmonary 
 artery there is only a single row ; this is also the case in all other 
 Amniota. In Crocodiles the right atrio-ventricular aperture is 
 guarded by a large muscular valve on the right (outer) side of the 
 aperture. 
 
 The sinus venosus, which even in the Amphibia especially 
 Anura shows indications of becoming sunk into the right atrium, 
 is now usually no longer recognisable as a distinct chamber ex- 
 
 1 A small aperture of communication between the two aortic roots, the foramen 
 Panizzce, exists in Crocodiles. 
 
VASCULAR SYSTEM 
 
 315 
 
 ternally (Figs. 254256). It becomes partially divided into two 
 portions by a septum ; and the left precaval, opening on the left of 
 
 D.Cti. 
 
 FIG. 256. 
 
 From the dorsal side. 
 
 FIG. 255. 
 
 FIG. 255. HEART OF A YOUNG Crocodilu* niloticus. 
 
 (After Rose). 
 
 Tr.cc, common carotid ; S.s, S.d, subclavian arteries ; A.s and A.d, left and right 
 aortic arches ; A.m, mesenteric artery ; L. V.h, fi. V.h, left and right atria ; 
 V.c.c, coronary vein. Other letters as in Fig. 244. 
 
 FIG. 256. HEART OF Crocodilus niloticus. From the right side. (After Rose). 
 Part of the wall of the right atrium is removed. 
 
 O.a.v, atrio- ventricular aperture ; Va.d and Va.s, the two sinu-auricular valves, 
 the white line between which is the margin of the sinu-atrial septum. Other 
 letters as in Figs. 244 and 245. 
 
 this septum, may appear to enter the right atrium independently 
 (e.g., Snakes,) The pulmonary veins unite into a single trunk 
 before entering the left atrium. 
 
 Birds and Mammals. In these Classes, the atrial and ventri- 
 cular septa are always complete, and there is no longer any mixture 
 
316 COMPARATIVE ANATOMY 
 
 of the arterial and venous blood. The muscular walls of the ventricle 
 are strongly developed and very compact. This is particularly the 
 case in the left ventricle, on the inner wall of which the papillary 
 muscles are well developed : the left ventricle is partially surrounded 
 by the right, the cavity of the latter having a semilunar transverse 
 section, and its walls being much thinner than those of the former 
 (Fig. 258). 
 
 In both Birds and Mammals the blood from the head and body 
 passes by means of the precavals and postcaval into the right 
 
 Ao. 
 
 \-MKl 
 
 FIG. 257. HEART OF GOOSE (Anser vtilgaris), DISSECTED FROM THE RIGHT SIDE. 
 
 (After Rose.) 
 
 The right atrium and ventricle are cut open, and their walls reflected. S.a, 
 septum atriorum ; L. Vi, limbus Vienssenii a ridge arising from the ventral 
 wall of the right atrium ; the space between this and the septum atriorum is 
 known as the spatium intersepto-valvulare (comp. Figs. 254 and 255). F.a.*, 
 V.a.d, the two sinu-auricular valves, situated at the entrance of the postcaval ;. 
 MK, MK', muscular right atrio-ventricular valve ; Ao, aorta ; V.c.s.d, right 
 precaval ; V.c.c, aperture of coronary vein. 
 
 atrium, as does also that from the walls of the heart through the 
 coronary vein l (Figs. 257, 259, 260, B), and the sinus venosus 
 especially in Mammals is scarcely recognisable (Figs. 257, 230) : 
 the right atrium is separated from the right ventricle by means of 
 a well-developed valve. In Birds (Fig. 257) this valve resembles 
 that of Crocodiles, and is very large and entirely muscular, while in 
 most Mammals it consists of three membranous lappets (tricuspid 
 
 1 Coronary veins are present in most of the lower Vertebrates also (comp. e.g., 
 Fig. 255), and the heart is supplied with arterial blood by coronary arteries, usually 
 arising in Fishes from a hypobranchial artery connected with the efferent branchials 
 or subclavians, and in higher forms from the base of the aorta. 
 
VASCULAR SYSTEM 317 
 
 valve) to which are attached tendinous cords, 1 arising from the 
 papillary muscles. 
 
 In Birds the left atrio-ventricular aperture is provided with a 
 valve consisting of three membranous folds : in Mammals there are 
 only two folds, and the valve is therefore known as the bicuspid or 
 mitral ; three semilunar pocket-like valves are also present? at the 
 origins of the pulmonary artery and aorta in both Birds and 
 Mammals. 
 
 As regards the origin of the great vessels, Birds are distinguished 
 from Mammals by the fact that in them the right, while in Mammals 
 
 Y.C.S.S. 
 
 FIG. 258. FIG. 259. 
 
 FIG. 258. TRANSVERSE SECTION THROUGH THE VENTRICLES OF Grits cinerea. 
 Vd, right, and Vy, left ventricle ; S, septum veiitriculorum. 
 
 FIG. 259. HEART or Ornithorhynchm paradox u*. From the dorsal side. 
 
 (After Rose. ) 
 
 V.c.s.8, V.c.x.d, precaval veins ; V.c.i, postcaval ; V.c.c, coronary vein ; V.c.s.s, 
 coronary sinus ; L.v, pulmonary veins ; Ao, aorta ; /*.*, P.d, pulmonary 
 arteries ; 1?. V.L, right atrium ; S.p.i, Spatium intersepto-valvulare. 
 
 the left aortic arch persists (Fig. 243, E, F) ; the corresponding arch 
 of the other side in both cases gives rise to part of the subclavian 
 artery. Thus in both Birds and Mammals there is only a single 
 aortic arch. As in Amphibians, the posterior arterial arch gives 
 rise to the pulmonary artery. The pulmonary veins, two from 
 each lung, open close together into the left atrium (Fig. 259). 
 
 Amongst the more important points in the development of the 
 heart may be mentioned the fact that in the embryo the two atria 
 communicate with one another secondarily by means of the foramen 
 ovale, through which the blood from the postcaval passes into the 
 left ventricle (Fig. 260). This foramen closes up when the lungs 
 
 1 There are no chordae tendinese in Monotremes, the heart of which in many 
 respects resembles that of the Sauropsida. 
 
318 
 
 COMPARATIVE ANATOMY 
 
 come into use, but its position can still be recognised as a thin area 
 (fossa oi'alis) in the atrial septum, surrounded by a fold (annulus 
 ovalis). Extending from this to the base of the postcaval and right 
 precaval respectively are two folds, known as the Eustachian and 
 
 F.O.V.L.V. 
 
 V.c.s. 
 
 FIG. 260. HEART OF HUMAN FOETUS (8ra MONTH). A, From the right, and B, 
 from the left side. (After Rose.) The walls of the atrium and ventricle are 
 partly removed in each figure. 
 
 Va.s, left sinu-auricular valve, fused with the septum atriorum (8. a, V.a.f) ; 
 Va.Th, Thebesian valve, in direct connection with the Eustachian valve 
 (Va.E) ; L. F, left atrium ; F.o.v, foramen ovale ; V.c.s, left precaval ; V.c.i, 
 postcaval; A.o, aorta ; P, P.d, P.s, pulmonary artery; DB, ductus Botalli 
 (ductus arteriosus) ; L.v. pulmonary vein ; V.c.c, coronary vein. 
 
 Thebesian valves (Fig. 260, A) ; these represent the remains of the 
 right sinu-auricular valve, and serve in the embryo to conduct the 
 blood from the right atrium into the left. 
 
 Great variations are seen in the mode of origin of the carotids 
 and subclavians from the arch of the aorta in Mammals. Thus 
 
 E 
 
 FIG. 261. FIVE DIFFERENT MODES OF ORIGIN OF THE GREAT VESSELS FROM 
 THE ARCH OF THE AORTA IN MAMMALS. 
 
 Ao, aortic arch ; tb, tbc, brachiocephalic trunk ; c, carotids ; s, subclavians. 
 
 there may be a Irachiocephalic or innominate trunk on either side 
 (Fig. 261, A) ; or an unpaired common brachiocephalic, from which 
 the carotid and subclavian of one or both sides arise (B, C, E) ; or, 
 
ARTERIAL SYSTEM 319 
 
 finally, a common trunk of origin for the carotids, the subclavians 
 arising independently on either side of it (D). 
 
 ARTERIAL SYSTEM. 
 
 The essential relations of the carotid arteries, dorsal aorta, and 
 pulmonary arteries, as well as the embryonic vitelline arteries, have 
 already been dealt with (pp. 301-305, Figs. 242, 264, &c.). An external 
 carotid and an internal carotid arise on either side independently from 
 the anterior efferent branchial arteries in Fishes and Dipnoans, but 
 from the Amphibia onwards these vessels are formed by the bifurca- 
 tion of each common carotid. In these higher types, the internal 
 carotid passes entirely into the cranial cavity, and supplies the brain 
 with blood, while the external carotid goes to the external parts of 
 the head (face, tongue, and muscles of mastication). 
 
 The origin of the subclavian artery, which supplies the anterior 
 extremity, is very inconstant, being sometimes symmetrical, some- 
 times asymmetrical. It arises either in connection with the 
 posterior efferent branchial vessels, or from the roots or main trunk of 
 the aorta (Figs. 262-264, &c.). Extending outwards towards the free 
 extremity, the subclavian passes into the brachial artery, from 
 which a dorsal and a ventral branch arise, and these subdivide again in 
 the lirnb. 
 
 From the dorsal aorta, in which a thoracic and an abdominal 
 portion can be distinguished in Mammals in addition to the caudal 
 portion, arise parietal (intercostal, lumbar}, and cceliac, mesenteric, 
 and urinogenital arteries, supplying the body-walls and viscera re- 
 spectively. These all vary greatly both in number and relative 
 size ; thus, for instance, there is sometimes a single cceliaco-mesen- 
 teric artery (Fig. 262), sometimes a separate cceliac, and one or more 
 mesenteric arteries (Fig. 264) ; x the renal and genital arteries also 
 vary in number and arrangement. All the branches of the dorsal 
 aorta, however, present primarily an approximately metameric 
 character, their number becoming more or less reduced owing to a 
 concentration of the vessels, which is more marked in short-bodied 
 than in long-bodied Vertebrates. 
 
 The aorta is continued posteriorly into the caudal artery, which 
 usually lies within a ccelomic canal enclosed by the ventral arches of 
 the vertebra? (Figs. 262-264) ; the degree of its development is 
 naturally in correspondence with the size of the tail. In cases 
 where the latter is rudimentary, as in Anthropoids and Man for 
 instance, the caudal aorta is spoken of as the median sacral artery, 
 and the aorta here appears to be directly continued, not by it, but 
 
 1 The cceliac typically supplies the stomach, liver, and spleen ; one or more 
 anterior mesenteries the whole intestine with the exception of the rectum, as well 
 as the pancreas ; and a posterior mesenteric the rectum. 
 
COMPARATIVE ANATOMY 
 
 <fe 
 
 -Arts 
 
 FIG. 262. THE ARTERIAL SYSTEM or Salamandra macnlosa. 
 
 RA, roots of the aorta ; Ao, Ao, dorsal aorta ; Sc, subclavian artery, from which 
 the cutaneous artery (Cu) arises : the latter anastomoses posteriorly with the 
 epigastric artery E ; Ov, ovarian arteries ; Cm, coeliaco-mesenteric ; H, 
 hepatic artery ; /, 7, /, anterior mesenteric arteries passing to the small 
 intestine ; M, M, posterior mesenteric arteries ; R, R, renal arteries ; I/r, 
 common iliac; Cr, crural artery; Hy, hypogastric artery; A, A, vesical 
 (allantoic) arteries; Aoc, caudal aorta; P, pharynx and oesophagus; m, 
 stomach ; p, pancreas ; /, liver ; d, d, small intestine ; e, rectum ; /, urinary 
 bladder ; Cl, cloaca. 
 
ARTERIAL SYSTEM 
 
 321 
 
 FIG. 263. THE ARTERIAL SYSTEM OF Emys europwa. 
 
 Tr, trachea ; Br, Br, the two bronchi ; m, stomach ; d, d, small intestine ; e, 
 large intestine ; Ap, pulmonary artery ; Cac, common carotids, with 
 tracheal and oesophageal branches (Tr, Oe);Sc, subclavian artery; Ver, vertebral 
 artery ; RA, roots of the aorta ; Ao, dorsal aorta ; Co, Co 1 , and Me, coeliaco- 
 mesenteric artery, which here arises as a bundle of separate vessels ; UG, 
 urinogenital arteries ; Cr, crural artery ; E, epigastric artery ; Is, sciatic 
 artery ; MB, posterior mesenteric arteries ; C, caudal aorta. 
 
 Y 
 
322 COMPARATIVE ANATOMY 
 
 by the common iliac arteries, which pass outwards into the pelvic 
 region. 
 
 Each common iliac artery becomes divided into an internal 
 iliac, or hypogastric, supplying the viscera of the pelvis, and 
 derived from the -proximal portion of the embryonic allantoic 
 artery, and an external iliac, which is continued into the crural or 
 femoral and supplies the hinder extremity (Fig. 262). In some 
 cases the internal and external iliacs come off separately from the 
 aorta (Fig. 263). The function of the femoral may be largely taken 
 by a sciatic artery arising separately from the aorta (Birds). The 
 main vessels again branch out in the limb. 
 
 VENOUS SYSTEM. 
 
 Fishes. Taking the Elasmobranchii more particularly into 
 consideration, a few of the more important facts as regards the 
 development of the veins must first be considered (comp. p. 301). 
 
 The first veins to appear in the embryo are the paired omphalo- 
 mesenteric veins, which bring back the blood from the surface of 
 the yolk and from the walls of the gut (Fig. 265, 1, II). The vessels 
 from the former region are known as vitelline veins, while those 
 from the latter give rise to siibintestinal veins (III VII), 
 running beneath the embryonic intestine, which primarily extends 
 into the caudal region as the post-anal gut. On the disappearance 
 of the latter, the posterior part of the subintestinal vessels gives 
 rise to the caudal vein, which now lies directly beneath the caudal 
 aorta and loses its direct connection with the anterior part (VIII 
 XII). As the liver is gradually developed, the main trunk of the 
 left omphalo-mesenteric vein breaks up into capillaries within this 
 organ, and these again unite anteriorly, opening into the proximal 
 ends of both omphalo-mesenteric veins. The latter thus give rise 
 to the hepatic veins, which open into the sinus venosus (or precaval, 
 e.g., in Cyclostomes). New vessels from the various parts of 
 the alimentary canal (gastric, splenic, and mesenteric veins) are 
 gradually developed, the pre-caudal portion of the subintestinal 
 vein becoming of minor importance ; all these vessels unite to 
 form what is now known as the hepatic portal vein, and thus 
 pour their blood through the capillaries of the liver (Figs. 270, 
 264268). 
 
 Anteriorly to the heart, a paired precaval vein (ductus Cuvieri) 
 is developed (Figs. 264 268), and opens into the sinus venosus. This 
 is formed, on either side, by the confluence of an anterior and a, posterior 
 cardinal vein, the former bringing back the blood from the head 
 (external and internal jugular veins), 1 and the latter from the body, 
 
 1 A single or paired inferior jugular from the ventral part of the head .may 
 also be present (Fig. 266). 
 
VENOUS SYSTEM 
 
 323 
 
 in which it runs on either side of the aorta, between it and the 
 kidneys. A subclaman vein from the pectoral fin also opens into 
 
 Y 2 
 
324 
 
 COMPARATIVE ANATOMY 
 
 Os 
 
 -Cdv 
 
 FIG. 265. DIAGRAM OF STAGES IN THE DEVELOPMENT OF THE VEINS IN ELASMO- 
 ERANCHS. (I XI after Rabl, XII after F. Hochstetter. ) 
 
 Ca, Cp, anterior and posterior cardinal veins ; Cdv, caudal vein ; D,D, vitelline 
 veins ; DC, precaval vein or sinus ; Cl, region of the cloaca ; H, sinus 
 venosus of heart ; J, subintestinal vein ; Jr. V, interrenal vein ; Lb, hepatic 
 veins ; **, hepatic sinus ; Npf, renal portal system ; VP, hepatic portal vein ; 
 Vpo, capillaries of the hepatic portal system ; f, cardinal sinus ; Sbc, sub- 
 clavian vein ; Os, Od, left and right oinphalo-mesenteric veins. 
 
 the precaval sinus or proximal end of the posterior cardinal 
 vein. 
 
 The caudal vein usually bifurcates posteriorly to the cloaca, each 
 branch passing along the outer side of the corresponding kidney, 
 
VENOUS SYSTEM 
 
 325 
 
 Card.ant[Jug} 
 
 SEV- 
 
 FIG. 266. For description see next page. 
 
326 COMPARATIVE ANATOMY 
 
 FIG. 266. DIAGRAM OF THE VEINS OF AX ELASMOBRANCH. 
 
 H, heart ; Duct. Cur, precaval sinus ; Card.ant (Jug), anterior cardinal (jugular) ; 
 the inferior jugular is seen nearer the middle line ; Subcl, subclavian ; Seit. 
 V, lateral vein, which arises from a venous network in the region of the 
 cloaca (Ven.Cl.J3), from one or more cutaneous veins of the tail (Cut. V), 
 from the veins of the body-walls, and from those of the pelvic fins 
 (HEV) ; Caud.v, caudal vein, which divides into two renal portals, A, A 1 , 
 at the posterior end of the kidneys (N) : from these arise the adve- 
 hent veins of the renal portal system (V.adv) ; V.rev, revehent renal 
 veins, from which the posterior cardinals (CV) arise; Card.V.S, cardinal 
 sinus, communicating with its fellow in the middle line ; V.port, hepatic 
 portal vein, receiving its blood from the intestine (ED), stomach (Mg), and 
 oesophagus (Oes.V), and anastomosing with the lateral vein posteriorly, and 
 with the cardinal sinus anteriorly ; Gen. V, genital veins ; L. V.S, hepatic 
 sinus ; Leb, liver. 
 
 and giving off advehent vessels into the latter (Figs. 264, 265, IX 
 XII, 266-268). These divide up into capillaries, forming a renal 
 portal system, the capillaries again uniting to form revehent veins 
 which open into the posterior cardinals. Thus the typical condition 
 of the veins seen in adult Fishes is reached, and only a few of the 
 more important modifications can be mentioned here. 
 
 In Cyclostomes and Elasmobranchs, the anterior part of the 
 subintestinal vein still persists as a small vessel running within 
 the spiral valve of the intestine. In the latter Order, many of the 
 veins (e.g., precavals, anterior and posterior cardinals, inferior jugulars, 
 hepatic and genital veins) enlarge to form capacious sinuses, and 
 a large lateral vein (Figs. 264, 266), running in the body-walls either 
 close to the skin or just external to the peritoneum, opens into each 
 precaval or posterior cardinal. This probably corresponds to the 
 vein of the primary lateral-fin folds (p. 104). 
 
 A renal portal system is said to be absent in Cyclostomes, and 
 is inconstant and very variable amongst Ganoids and Teleosts : in 
 many instances the caudal vein communicates directly with one or 
 with both posterior cardinals, and in the former case the other 
 cardinal shows a tendency to become reduced in size : a similar 
 reduction occurs in many of the forms to be described next. 
 
 Dipnoi. The chief point of interest as regards the veins of 
 Dipnoans (Fig. 267) is the presence of a large unpaired postcaval 
 vein, derived in part from the posterior cardinal, and comparable to 
 that of the Amphibia and Amniota. A renal portal system is 
 present, and the blood from the kidneys is collected into two veins 
 having the relations of posterior cardinals. Only the left of these 
 however, opens anteriorly into the corresponding precaval, the 
 right, which is much the larger of the two, passing along the 
 dorsal border of the liver to open independently into the sinus 
 venosus in the middle line. The renal portion of this vein is 
 evidently homologous with the corresponding part of the posterior 
 cardinal, the anterior portion of which can no longer be recognised. 
 
VENOUS SYSTEM 
 
 327 
 
 A 
 
 -Vsbc 
 
 FIG. 267. DIAGRAM OF THE VENOUS SYSTEM OF Protopterus annectens. 
 (After W. N. Parker.) 
 
 ;, ventricle ; at, atrium ; p, pericardium ; ca, conus arteriosus ; Ji, Je, internal 
 and external jugular ; Vsbc, subclavian ; DC, DC', precaval veins ; Cp, post- 
 caval ; Vh, Vh, hepatic veins ; L, liver; G.B, gall-bladder ; G.G, bile-duct ; 
 M, "stomach"; Da, intestine; L.G, lymphoid organ in the walls of the 
 stomach, the blood from which passes into the hepatic portal veins (Vpo, 
 
 po; 
 
 is connected by anastomoses (ans) with the postcaval (Cp) in the region of 
 
 the kidneys, 
 
328 COMPARATIVE ANATOMY 
 
 Thus the postcaval is made up of a posterior or renal portion, and 
 of an independently developed anterior or hepatic portion. 
 
 In Ceratodus, the posterior cardinal and postcaval are directly 
 continuous with the caudal vein, and the renal portal, receiving 
 branches from the posterior end of the body, arises from the iliac vein, 
 which also gives off a pelvic branch. The latter unites with its 
 fellow in the middle line to form a median abdominal vein, com- 
 parable to that of the Amphibia, and opening into the sinus 
 venosus. 
 
 The two pulmonary veins unite into a single trunk before 
 opening into the left atrium (p. 309). 
 
 Amphibia. A large postcaval vein arises in essentially the 
 same manner as in the Dipnoi, its renal section being formed by 
 the fusion of the two posterior cardinals in this region. The 
 hepatic portion apparently arises in part from the right omphalo- 
 mesenteric vein, and in part independently, while the hepatic 
 portal vein is developed from the left omphalo-mesenteric. The 
 postcaval receives blood from the kidneys and generative organs, 
 as well as indirectly from the posterior extremities, body- walls, and 
 tail (when present). The anterior part of loth posterior cardinals 
 persists in Urodeles and in Bombinator as the paired azygos vein, 
 and this may exceptionally be present on one or both sides in other 
 Anurans. It communicates with the corresponding precaval 
 (Fig. 268). 
 
 A renal portal system is present, and is formed, as in Fishes, 
 by the bifurcation of the caudal vein, which is wanting in adult 
 Anura ; into the renal portal open the veins from the hind-limb, 
 and vessels from the body-wall often also communicate with it. 
 The blood from the kidneys passes into the postcaval. Connecting 
 the right and left renal portals (or femorals) is a transverse pelvic 
 vein, from which, in the medio-ventral line of the body, an abdominal 
 or epigastric vein arises,as in Ceratodus: this is primitively'paired,and 
 corresponds genetically with the lateral veins of Elasmobranchs ; 
 it extends forwards in the ventral body-wall into the liver, in 
 which it breaks up into capillaries, becoming secondarily connected 
 by anastomoses with the hepatic portal vein (Fig. 268). The ab- 
 dominal vein receives blood from the cloaca, bladder, and body- 
 walls. In Urodeles remains of the subintestinal vein also open 
 into the hepatic portal system. 
 
 The arrangement of the anterior cardinals (external and inter- 
 nal jugulars) is essentially similar to that seen in Fishes and 
 Dipnoans. 
 
 Amniota. The section of the right posterior cardinal vein in 
 the region of the embryonic kidney (mesonephros, p. 341) gives 
 rise, as in the Dipnoi, to the hinder part of the postcaval : the 
 hepatic section of the latter arises as in Amphibia. In the Saur- 
 opsida, the anterior portions of both posterior cardinals disappear, 
 
Card. ant. (Jug] 
 
 Sulcl 
 
 V.iln 
 
 FIG. 268. DIAGRAM OF THE VENOUS SYSTEM OF Salamandm maculoaa. 
 
 Cand. V, caudal vein, which bifurcates at the posterior end of the kidneys (N, N) 
 to form the renal portal system (Nier.Pft.Kr) ; V.adv, V.rev, advehent and 
 revehent renal veins ; V,iliaca, femoral vein, which divides into an anterior 
 (ft) and a posterior (f) branch : the latter opens into the renal portal, and 
 the former (pelvic vein) unites with its fellow to form the abdominal vein 
 (Abd. V), and also receives vessels (*) from the cloaca, bladder, and posterior 
 part of the intestine. V.Cava inf. pars anter, and V. Cava inf. pars poster, 
 anterior and posterior sections of the postcaval ; Card. ant (Jug), and Card, 
 post (Azyg), anterior and posterior cardinal veins (i.e., the jugular 
 and azygos). Subcl, subclavian vein ; Duct.Cuv, precaval ; H, heart ; Z>, Z), 
 alimentary canal, from which the hepatic portal vein ( V. port] arises ; Lg, 
 longitudinal vein of the intestine; Lpft.Kr, hepatic portal system; L. V, 
 hepatic vein. 
 
330 
 
 COMPARATIVE ANATOMY 
 
 and are replaced by vertebral veins, while in Mammals they persist 
 as the azygos veins. An anastomosis is formed between these, and 
 eventually the anterior part of the left disappears, the blood from 
 both sides passing into the right azygos (hemiazygos), which opens 
 into the right precaval (Figs. 269 and 270). 
 
 The anterior cardinals give rise, as in lower Vertebrates, to the 
 jugulars, which, as well as the subclavians and vertebrals or azygos, 
 
 FIG. 269. DIAGRAM SHOWING THE RELATIONS OF THE POSTERIOR CARDINAL 
 
 AND POSTCAVAL VEINS IN A, THE RABBIT, AND B, MAN. (After Hoch- 
 
 stetter). 
 
 V.r.d, V.r.s, renal veins ; V.d.s.e, common iliac vein ; VI. I, lumbar vein ; V.c.i, 
 postcaval ; V.c.p.d, V.c.p.s, right and left posterior cardinals ; V.il.int.comm, 
 common internal iliac vein. 
 
 open into the precavals. In Reptiles, Birds, Monotremes, and 
 Marsupials, as well as in many Rodents, Insectivores, Bats, and 
 Ungulates, both precavals persist throughout life ; but in other 
 Mammals the main part of the left disappears, all the blood from 
 the head and anterior extremities passing into the right. The 
 coronary veins open into the base of the left precaval (coronary 
 sinus, Fig. 259). 
 
VENOUS SYSTEM 
 
 331 
 
 A renal portal system occurs in. connection with the embryonic 
 kidney in all Sauropsida, and traces of it can also be recognised 
 in embryos of Echidna. In adult Reptiles, renal portal veins give 
 off branches into the permanent kidney (metanephros, p. 346) : in 
 
 IT 
 
 FIG. 270. DIAGRAM ILLUSTRATING THREE STAGES IN THE DEVELOPMENT OF 
 THE HEPATIC PORTAL SYSTEM. (See next page for c.) 
 
 H, heart ; Sv, sinus venosus ; DC, DC, precavals ; Ci, postcaval ; L, liver ; Om, 
 Om l , Om", the three sections of the omphalo-mesenteric vein (the first still 
 shows its originally paired nature at ft : in stage B, the second section of this 
 vein, which passes through the liver, disappears, so that Om and Om 2 are 
 only connected by capillaries : in stage C, the first section (Om) has quite 
 disappeared, and the umbilical vein (Umb] has become developed); DA, 
 ductus venosus ; *, connection of the umbilical vein with the capillaries of 
 the liver; Vr, revehent veins; Vad, advehent veins; Mes., mesenteric 
 vein, which later gives rise to the hepatic portal ( V.port), receiving 
 blood from the alimentary canal (D) ; Az., azygos ; J7, iliac vein; JY, 
 kidney. 
 
 Birds only a slight indication of such a renal portal system exists, 
 and in Mammals it is entirely wanting. 
 
 As in Fishes, the first veins to appear in the embryo are the 
 omphalo-mesenteric veins (Fig. 270, A), bringing back the blood from 
 
332 
 
 COMPARATIVE ANATOMY 
 
 the yolk-sac, and uniting into a single trunk before opening into 
 the heart. As the liver becomes developed, a portal circulation 
 arises, and the main trunk of the vein, where it passes through the 
 liver, disappears. In the meantime, the coeliac and mesenteric 
 veins have become developed, and all the blood from them, as well 
 as from the vitelline veins, now passes through a common trunk, 
 the hepatic portal vein, into the capillaries of the liver, whence it 
 
 FIG. 270, c. Reference to lettering on previous page. 
 
 reaches the sinus venosus through the hepatic veins. The vitelline 
 veins gradually disappear as the yolk-sac becomes reduced. 
 
 In addition to these vessels, the umbilical vein must also be 
 mentioned. This vessel is originally paired, and corresponds 
 genetically to the lateral veins of Elasmobranchs and to the 
 abdominal or epigastric vein of Ceratodus and Amphibians. It 
 is situated originally in the body-walls, and comes into rela- 
 tion with the allantois (pp. 9 and 387), opening eventually into the 
 
LYMPHATIC SYSTEM 333 
 
 postcaval : as the allantois increases in size, it brings back the 
 oxygenated blood from this organ (i.e., from the placenta in the 
 higher Mammalia). The right umbilical vein, however, early be- 
 comes obliterated, and the left comes into connection with the 
 capillaries of the liver, its main stem in this region disappearing 
 (Fig. 270, B). Thus the blood from the allantois has to pass through 
 the capillaries of the liver before reaching the heart. In the course 
 of development, however, a direct communication is formed be- 
 tween the left umbilical, vein and the remains of the fused vitelline 
 veins, and this trunk is known as the ductns venosus (Fig. 270, c). 
 On the cessation of the allantoic (or placental) circulation, the 
 ductus venosus becomes degenerated into a fibrous cord, so that 
 all the portal blood has to pass through the capillaries of the liver. 
 The intra-abdominal portion of the umbilical vein persists 
 throughout life as the epigastric vein in Reptiles and in Echidna, 
 but disappears in Birds and in other Mammals. 
 
 The mode of development of the veins of the extremities is essentially 
 similar in all the Amniota, and at first resembles that occurring in 
 Amphibia, though later on considerable differences are seen in these two 
 groups, more especially as regards the veins of the digits. 
 
 Retia Mirabilia. 
 
 By this term is understood the sudden breaking-up of an arte- 
 rial or venous vessel into a cluster of fine branches, which, by 
 anastomosing with one another, give rise to a capillary network ; 
 the elements of this network may again unite to form a single 
 vessel. The former condition may be described as a unipolar, the 
 latter as a bipolar rete mirabile. If it is made up of arteries or of 
 veins only, it is called a rete mirabile simplex ; if of a combination 
 of both kinds of vessels, it is known as a rete mirabile duplex. 
 
 The retia mirabilia serve to retard the flow of blood, and thus 
 cause a change in the conditions of diffusion. They are extremely 
 numerous throughout the Vertebrate series, and are found in the 
 most varied regions of the body, as, for instance, in the kidneys 
 (glomeruli, p. 345) where their above-mentioned function is most 
 clearly seen ; on the ophthalmic branches of the internal carotid ; 
 on the vessels of the air-bladder in Fishes (p. 280) ; along the 
 intercostal arteries of Cetacea ; on the portal vein ; and along the 
 caudal portion of the vertebral column in Lizards. 
 
 LYMPHATIC SYSTEM. 
 
 In Fishes, Amphibians, and Reptiles, but more particularly in 
 the first-named Class, lymph vessels (p. 299) are often not 
 plainly differentiated, and occur mainly along the great blood- 
 
334 COMPARATIVE ANATOMY 
 
 vessels, as well as on the bulbus arteriosus and ventricle, lying 
 in the connective-tissue surrounding these structures. Numerous 
 independent lymphatic vessels may, however, also be present, 
 arising from a capillary network under the skin, and extending into 
 the intermuscalar septa ; the intestinal tract and the viscera are also 
 generally provided with definite lymph-vessels in the Amphibia 
 and Amniota. 
 
 Contractile lymph-hearts may be present in connection with the 
 vessels. They occur in Fishes, but are much better known in 
 Amphibians, Reptiles, and Bird-embryos. Thus, in Urodeles, nume- 
 rous lymph-hearts are present under the skin along the sides of 
 the body and tail, at the junction of the dorsal and ventral body- 
 muscles ; in other Amphibians they are either confined to the poste- 
 rior end of the body (pelvic region), or, as in the Frog, are present 
 also between the transverse processes of the third and fourth 
 vertebrae. In Reptiles posterior lymph-hearts only are present, 
 and are situated at the boundary of the trunk and tail regions, 
 close to the transverse processes or ribs. Similar structures are not 
 known to be present in Mammals. 
 
 Large lacunar lymph- sinuses are present under the skin of tail- 
 less Amphibia, and the skin is thus only loosely attached to the un- 
 derlying muscles. These subcutaneous lymph- sinuses are connected 
 with those of the peritoneal cavity. Amongst the latter, the sub- 
 vertebral lymph-sinus is of great importance in Fishes, Dipnoans, 
 and Amphibians ; it surrounds the aorta and is connected with the 
 (mesenteric) sinus lying amongst the viscera, into which the 
 lymphatic vessels of the intestine open. In Fishes and Dipnoans 
 there is also a large longitudinal lymphatic trunk lying within the 
 spinal canal. 
 
 As already mentioned, the higher we pass in the animal series 
 the more commonly are lymphatic trunks with independent walls to 
 be met with. From Birds onwards a large longitudinal subverte- 
 bral trunk (the thoracic duct) is always present. In Mammals this 
 arises in the lumbar region, where it is usually dilated to form the 
 cisterna or receptaculum chyli ; it receives the lymph from the 
 posterior extremities, the pelvis, and the urinogenital organs, as 
 well as the lacteals, or lymphatics of the intestine. In Mammals it 
 communicates anteriorly with the left, and in Sauropsida with both 
 left and right precaval veins. The lymphatics of the head, neck, 
 and anterior extremities open into the same veins. 
 
 The lymphatic vessels of Birds and Mammals are, like certain 
 of the veins, provided with valves, the arrangement of which allows 
 the lymph stream to pass in one direction only, i.e., towards the 
 veins. 
 
 The lymph, as already mentioned (p. 299), consists of two 
 elements, a fluid (plasma) and cells (lymph-corpuscles, leucocytes) ; 
 and similar cells are present in the lymphoid or adenoid tissue which 
 occurs beneath the mucous membrane in various parts of the body 
 
LYMPHATIC SYSTEM 335 
 
 (e.g., alimentary canal, bronchi, conjunctiva, urinogenital organs) 
 and is particularly abundant in Fishes, Dipnoans, and Am- 
 phibians (pp. 267, 352, 363). 
 
 The migration of the amoeboid leucocytes to the surface (p. 267) is due 
 to various causes. It may simply result in getting rid of superfluous 
 material, or may be of considerable importance in removing broken-down 
 substances and harmful bodies (e.(/., inflammatory products, Bacteria), the 
 particles being ingested by leucocytes (hence often called phagocytes) 
 before the latter are got rid of. 
 
 The mass of lymphoid tissue 011 the heart of the Sturgeon, and possibly 
 also the so-called fat-bodies (corpora adiposa) of Amphibia and Reptilia 
 (pp. 368, 370), and the "'hibernating gland" of certain Rodents, may be 
 placed in this category ; they consist of lymphoid and fatty tissue, and serve 
 as stores of nutriment. 
 
 The agglomeration of a number of lymphoid follicles gives rise 
 to those structures which are spoken of as "lymphatic glands 9 ' 
 or adenoids. These are always interposed along the course of 
 a lymphatic trunk so that afferent and efferent vessels to each 
 can be distinguished. They probably appear first in Birds, and 
 are most numerous in Mammals, where they are present in 
 abundance in various regions of the body ; they differ greatly in 
 size. 
 
 The spleen which is present in almost all Vertebrates, is 
 closely related to these structures. It corresponds to a specially 
 differentiated portion of a tract of lymphoid tissue primarily 
 extending all along the alimentary canal, and in Protopterus 
 it still remains enclosed within the walls of the stomach 
 (Fig. 209). In other Vertebrates it is situated outside the 
 walls of the canal, but even then may extend along the greater 
 part of the latter (e.g., Siren). Usually, however, either the 
 proximal or the distal portion of it undergoes reduction, and the 
 organ is generally situated near the stomach, though it is occa- 
 sionally met with in other regions of the intestinal tract, as, for 
 instance, at the commencement of the rectum (Anura, Chelonia). 
 In some cases (e.g., Sharks) it is broken up into a number of 
 smaller constituents. 
 
 The tonsils are also adenoid structures. They are most 
 highly developed in Mammals, where they give rise to a paired 
 organ lying on either side of the fauces that is, in the region 
 where the mouth passes into the pharynx, and usually also to a 
 mass situated more posteriorly on the walls of the pharynx 
 itself (pharyngeal tonsils); the latter are phylogenetically the 
 older organs and are present in Reptiles, Birds, and most Mam- 
 mals. 1 The tonsils consist of a retiform (adenoid) connective- 
 tissue ground-substance enclosing a number of lymph-corpuscles, 
 which are arranged in so-called follicles, and are capable of mi- 
 grating to the surface. 
 
 1 Tonsil-like organs are also present in Amphibians. 
 
336 COMPARATIVE ANATOMY 
 
 New leucocytes are continually formed in the marrow of the 
 bones, as well as in the lymphatic glands and spleen ; the spleen 
 is apparently also of importance in absorbing the broken-down 
 remains of the red blood- corpuscles. 
 
 MODIFICATIONS FOE, THE INTER-UTERINE NUTRI- 
 TION OF THE EMBRYO: FCETAL MEMBRANES. 
 
 I. ANAMNIA. 
 
 IN several Elasmobranchs the oviduct gives rise to glandular 
 villi which secrete a nutritive fluid, and in an Indian Ray (Ptero- 
 platea micrura) there are specially long glandular villiform pro- 
 cesses which extend in branches through the spiracles into the 
 pharynx of the embryos, of which there may be as many as three 
 in each oviduct. The gill-clefts of the embryos are in close appo- 
 sition, and there are no gill filaments (see p. 278). 
 
 In certain viviparous Sharks (viz., Mustelus Isevis and Carcha- 
 rias) the walls of the vascular yolk-sac become raised into folds or 
 villi, which fit into corresponding depressions in the walls of the 
 oviduct, t'he latter becoming very vascular. A kind of umbilical 
 placenta is thus formed, by means of which an interchange of nutri- 
 tive, respiratory, and excretory matters can take place between the 
 maternal and foetal blood-vessels. 
 
 Amongst viviparous Teleosts (comp. p. 360) various arrange- 
 ments for the nutrition of the embryo occur. In Zoarces 
 viviparus (and probably also in the Embiotocideae), the embryos 
 are retained in the hollow ovary, the empty follicles (corpora lutea) 
 of which give rise to extremely vascular villi, from which a serous 
 fluid containing blood- and lymph-cells is extruded into the cavity 
 of the ovary and thus surrounds the masses of embryos. These 
 swallow the fluid and digest the contained cells. In other forms 
 (e.g., viviparous Blennies, and Cyprinodonts), the embryos undergo 
 development within the vascular follicles, and are probably nour- 
 ished by diffusion; while in Anableps, villi are developed from the 
 yolk-sac, and these doubtless absorb the nutritive fluid from the 
 walls of the ovary. 
 
 In certain Amphibians which have no prelarval existence, in- 
 teresting modifications occur for nourishing the young until the 
 larval stage is passed. Thus in the Alpine Salamander (Salaman- 
 dra atra), a large number of ova (40 60) pass into each oviduct, 
 just as in the allied S. maculosa, in which the young are born as 
 gilled larvaa. Were this the case in S. atra, the young would be 
 carried away in the mountain streams and destroyed, and a curious 
 adaptive modification has therefore arisen in this form, in 
 
FCETAL MEMBRANES 337 
 
 which only one embryo (that nearest the cloaca) in each oviduct 
 undergoes complete development, remaining within the body of 
 the parent until the gills are lost and metamorphosis has taken 
 place. The other eggs break down and form a food-mass for 
 the survivors after their own yolk has become used up. Degene- 
 rative changes, moreover, take place in the epithelium of the ovi- 
 duct, and masses of red blood-corpuscles pass into the lumen of the 
 latter, undergo degeneration, and become mixed with the broken- 
 down yolk-masses, the resulting broth being swallowed by the 
 surviving young. After the birth of the latter, the uterine epithe- 
 lium becomes regenerated ; and thus a process occurs which some- 
 what resembles that of the formation of a decidua in placental 
 Mammals (p. 340). 
 
 II. AMNIOTA. 
 
 In all the Anmiota, as already mentioned (pp. 9 and 302), foetal 
 membranes, known as the amnion and allantois are developed, 
 the latter, or primary urinary bladder, represented only in rudi- 
 ment in the Amphibia (p. 259), being of great importance in con- 
 nection with respiration, secretion, and (in the higher Mammals) 
 nutrition of the embryo. 
 
 A glance at Fig. 8 will show that, owing to its mode of develop- 
 ment, the amnion 1 consists primarily of two layers ; an inner, the 
 amnion proper, and an outer or false amnion. The latter lies close 
 to the vitelline membrane, and forms the so-called serosa, or serous 
 membrane. As the allantois grows it extends into the space con- 
 tinuous with the ccelome between the true and false amnion, and 
 may entirely surround the embryo. 
 
 Amongst Reptiles, the eggs of the viviparous Lizard, Seps chal- 
 cides, are relatively poor in yolk, and this is compensated for by 
 the yolk-sac and allantois coming into close relation with the 
 walls of the oviduct, thus forming an umbilical and an allantoic 
 placenta, one at either pole of the embryo ; the latter of these is 
 the more important. Both foetal and maternal parts of the pla- 
 centae become extremely vascular, and thus the necessary inter- 
 change of materials can take place between the blood of the em- 
 bryo and mother. In Trachydosaurus and Cyclodus, as well as in 
 the Chelonia, a kind of umbilical placenta is apparently also 
 formed. . 
 
 The fact that a vascular yolk-sac (often known as the umbilical 
 vesicle) is present in placental Mammals, indicates that they are 
 descended from forms in which, like the Sauropsida, the eggs were 
 rich in yolk, and which were viviparous. This condition is 
 
 1 As the head enlarges and sinks downwards, it is at first surrounded by a 
 modification of the head fold (p. 9) consisting entirely of epiblast and called the 
 pro-amnion : this is afterwards replaced by the amnion. 
 
 Z 
 
338 
 
 COMPARATIVE ANATOMY 
 
 moreover retained in the Monotremes, and even in Marsupials 
 the ova are relatively large as compared with those of the higher 
 Mammalia. 
 
 As the amount of yolk gradually became reduced in the course 
 of phylogenetic development, close relations were set up between 
 the fcetal (allantoic) and maternal blood-vessels, the allantois 
 becoming closely applied to the serosa to form a chorion (Fig. 
 271) ; but that this condition was only very slowly evolved is 
 shown by the fact that, even at the present day, Mammals exist in 
 which it has not been reached. These (viz., Monotremes and most 
 Marsupials) are therefore known as Aplacentalia or Achoria, in 
 contradistinction to the higher Placentalia or Choriata. Moreover^ 
 
 in the Rodentia,Insectivora, Cheir- 
 optera, Carnivora, and Ungulata 
 more or less distinct indications 
 of an umbilical placenta, formed 
 in connection with the yolk-sac,. 
 can still be observed, and at a still 
 earlier stage the ova are nourished 
 by uterine lymph (compare p. 336). 
 In Monotremes and Marsu- 
 pials, both the yolk-sac and allan- 
 tois take part in respiration; in 
 the former the two are of equal 
 importance, while amongst the 
 latter the yolk-sac is solely or 
 mainly (Phalcolarctos) important 
 in this respect. In Perameles 
 obesula a further approach towards 
 the formation of a true allantoic 
 placenta is seen, the allantois 
 giving rise to small vascular villi. 
 In most Marsupials the allantois 
 serves merely as a urinary reser- 
 arise. voir, and in none of them does 
 
 it possess any important function 
 
 as an organ of nutrition, the young being born at a relatively early 
 stage, when they become attached to the teats of the mother, and 
 are then nourished by means of milk (see p. 288). 
 
 In the higher Mammals, the umbilical placenta has usually 
 only a very temporary importance, though in some cases (e.g.,, 
 Rodents) it probably takes some part in respiration and nutrition 
 during the whole uterine life. The allantois extends out from the 
 body of the embryo and becomes attached to the serous membrane 
 to form the chorion, from which numerous villi extend into the 
 uterine wall (Fig. 271). As both the latter and the allantois become 
 extremely vascular, the uterine and allantoic capillaries and 
 sinuses coming into close contact with one another, a complicated 
 
 FIG. 271. DIAGRAM or THE FCETAI, 
 MEMBRANES or A PLACENTAL 
 MAMMAL. (From Boas's Zoology.) 
 
 al, allantois; am, amnioii ; &, yolk-, 
 sac (umbilical vesicle) ; the outer- 
 most line represents the serous 
 membrane. The outer wall of 
 the allantois has united with the 
 serous membrane to form the 
 chorion from which branchial villi 
 
FCETAL MEMBRANES 339 
 
 allantoic placenta, arises, consisting of maternal and fcetal parts 
 (Fig. 9). Thus the embryo is supplied with the necessities for 
 existence during its comparatively long intra-uterine life. 
 
 Various forms of placenta are met with amongst the Placentalia. 
 The most primitive type is apparently that in which the allantois 
 becomes attached around the whole serosa, so that the resulting 
 chorion, from which the comparatively simple villi arise, are equally 
 distributed over the whole surface (Fig. 271). This form is known 
 as a diffused placenta, and is met with in Manis, the Suidse, Hippo- 
 potamuSi Tylopoda, Tragulidae, Perissodactyla, and Cetacea. 
 
 The next stage is characterised by the chorionic villi becoming 
 more richly branched, so as to present a greater superficial extent, 
 and at the same time being concentrated into definite and v 
 
 Ch orion ffffa e sss 
 
 A&1tfrl..--'j6jlw '~ r . ;' ,-,- , " ; ; ""^^~? 
 
 4*c** y *'* ^ i ^^ 
 
 ' .- *<,?. *t* 
 
 Ztttea Jes&cno*. ^ *<*> 
 
 ^ 
 
 % \ 
 
 FIG. 272. DIAGRAM TO ILLUSTRATE THE RELATIONS OF IHE FCETAL AND 
 MATERNAL VESSELS IN THE HUMAN PLACENTA, SHOWING CHORIONIC AND 
 MATERNAL VESSELS AND CAPILLARIES, VILLI (Zotten), AND DECIDUA. 
 (After Keibel.) 
 
 more or less numerous patches or cotyledons. Thus a polycoty- 
 ledonary placenta arises, such as is met with in most Ruminants, 
 some of which, such as Cervus mexicanus and the Giraffe, show an 
 interesting intermediate form of placenta between the diffuse and 
 the cotyleclonary. 
 
 The chorionic viili in these two types of placenta, even though 
 more or less branched, separate from the uterine mucous membrane 
 at birth, the latter not becoming torn away : these placentae are 
 therefore spoken of as non-deciduate. 
 
 A further complication is seen in the forms of placenta known as 
 thezonary, the dome- or bell- shaped, and the discoidal, in which the 
 connection between fcetal and maternal parts becomes much more 
 close, the villi giving rise to a complicated system of branches 
 within the uterine mucous membrane (Fig. 272). Thus the latter 
 
 z 2 
 
340 COMPARATIVE ANATOMY 
 
 becomes to a greater or less extent torn away at birth (decidua), 
 the placenta being therefore spoken of as deciduate. In these 
 cases, the placental part of the chorion does not extend all round 
 the embryo. In the zonary placenta only the two opposite poles 
 of the chorion are .more or less free from vascular villi, and this 
 girdle-like form occurs in the Carnivora, as well as in the Elephant 
 Hyrax, and Orycteropus. In Lemurs and Sloths, the placenta is 
 dome- or bell-shaped, while in Myrmecophaga, Dasypodidae (Arma- 
 dilloes), and Primates (Fig. 9) it forms a discoidal mass on the 
 dorsal side of the embryo (metadiscoidal form). The discoidal 
 placenta of Rodentia, Insectivora, and Cheiroptera has probably 
 not arisen, like that just mentioned, from a diffused type, but was 
 originally restricted to a discoidal area, owing to the umbilical 
 vesicle occupying a large surface of the chorion. 
 
 From the above description it is evident that the differences 
 in the form of the placenta are mainly those of degree, and that 
 the latter gives little indication of the systematic position of the 
 animal in question. 
 
 The histological structure of the placenta and the various modifications 
 seen in the maternal mucous membrane cannot be described here ; it is, 
 however, important to note that there is no direct communication between the 
 maternal and foetal blood, and that the maternal capillaries usually enlarge 
 to form sinuses, the walls of which become invaginated by the villi : 
 thus the latter are covered by an epithelium furnished by the maternal 
 tissues (Fig. 272). 
 
 In the course of development the embryo becomes more and 
 more folded off from the yolk-sac (Fig. 8), the stalk of which latter 
 and that of the allantois, enveloped by the base of the amnion, 
 together form the umbilical cord. At birth, the foetal membranes 
 are shed, the intra-abdorninal portion of the allantois persisting as 
 the urachus (comp. p. 358). 
 
I. URINOGENITAL ORGANS. 
 a, GENERAL PART. 
 
 The first traces of the urinary and generative organs of Verte- 
 brates arise on the dorsal side of the ccelome, right and left of the 
 aorta, and are more or less closely connected with one another, 
 both morphologically and physiologically. 
 
 The part of the urinogenital system first to arise is the paired 
 pronephros and its duct, the pronephric duct. This is the 
 most ancient and primitive excretory organ of Vertebrates ; it is 
 usually restricted to a few of the anterior body segments, close 
 behind the head, whence it is often known as the " head-kidney." 
 It originates primarily as a series of segmentally arranged 
 invaginations of the somatic mesoblast in the region of the ventral 
 section of the mesoblastic somites, these invaginations giving rise 
 to excretory tubules or nephridia (Figs. 273 and 274) ; secondarily, 
 however, in consequence of alterations in the relative rate of 
 growth of the parts, the tubules come to arise in connection with 
 the unsegmented body-cavity. Each tubule opens into the ccelome 
 by a ciliated funnel or nephrostome, and comes into relation with a 
 segmental blood-vessel which primarily connects the aorta with the 
 subintestinal vein. These vessels become coiled to form a rete 
 mirabile known as the glomus (Fig. 274). Primarily, as in Chseto- 
 pods, the tubules must have opened at the other end on to the 
 surface independently, through the ectoderm (Fig. 277, A, and comp. 
 Amphioxus, p. 348 and Figs. 219 and 277, A), but this condition is 
 no longer observable in the Craniata, in which they all communi- 
 cate with a longitudinal pronephric duct. The number of nephro- 
 stomes is in most cases not more than two or three. 
 
 The pronephric duct is apparently a later acquisition than the 
 pronephros itself. It first appears in the somatic mesoblast, 1 
 arising by the fusion of the peripheral ends of the pronephric 
 tubules to form a longitudinal collecting tube (Figs. 274, 277, B), 
 which extends back wards to open into the cloaca, thus establishing 
 a communication between the ccelome and the exterior. 
 
 1 In Elasmobranchs its origin can be traced to the epiblast. 
 

 
 
 .-d.pn. 
 
 G 
 
 1 
 
 .*i,l ms.s. 
 
 i -...V.S. 
 
 d.m. 
 d.ms. 
 
 hy.s. 
 
 ,-/ 
 
 II 
 
URINOGENITAL ORGANS 
 
 343 
 
 FIG. 273. A SERIES OF DIAGRAMMATIC FIGURES ILLUSTRATING THE ACCOUNT OF 
 THE COMPARATIVE MORPHOLOGY OF THE URINOGENITAL ORGANS OF THE 
 YERTEBRATA GIVEN IN THE FOLLOWING PAGES. 
 
 A, the pronephros stage of the Anamnia ; B, a later stage of the same ; C, the 
 urinogenital apparatus of the male Amphibian ; D, the same of the female ; 
 
 E, pronephros stage of the Anmiota, the mesonephros as yet rudimentary ; 
 
 F, urinogenital apparatus of the Amniota at a stage at which the sexes are 
 not differentiated ; G, urinogenital apparatus of male Amniota ; H, the same 
 of female Amniota. 
 
 p.n., pronephros; d.pn., duct of the pronephros; ms., the developing me- 
 sonephros ; ms.s, part of the mesonephros, becoming converted into the 
 epididymis and parovarium ; ms.r, vestiges of the mesonephros, the para- 
 didymis and the paroophoron ; f, rete and vasa efferentia testis ; ft, a 
 network homologous with these structures at the hilum of the ovary ; hy.s, 
 stalked hydatid ; ms.z, portion of the mesonephros which in Amphibians and 
 Elasmobranchs becomes the so-called pelvic kidney ; d.ms, duct of the 
 mesonephros, which in male Amphibians and Elasmobranchs becomes 
 (Fig. C) the urinogenital, and in females (Fig. D) the urinary duct. In the 
 male Amniota it gives rise to the seminal duct (Fig. G), and in the female to 
 Gartner's duct (Fig. H) ; r.s, the seminal vesicle, an outgrowth of the duct 
 of the mesonephros ; d.m., Miillerian duct, which in Mammals becomes 
 differentiated (Fig. H) into the Fallopian tube (fl ), the uterus (ut), and the 
 vagina (vg) vs, its abdominal aperture; hy, and u.m (Fig. G), unstalked 
 hydatids and uterus masculinus (vestiges, in the male, of the Miillerian duct, 
 d.m.) ; m.t., the definitive kidney or metanephros of the Amniota, said to 
 arise from the ureter (nr), itself an outgrowth of the mesonephric duct ; al, 
 allantois or urinary bladder ; su, urinogenital sinus ; p.g, genital prominence, 
 y.g, gonads, undifferentiated stage; ov, ovary; ts., testis; d, cloaca; al, 
 rectum ; p. a, abdominal pore ; g.c, Cowper's glands. 
 
 TABULATED RESUME OF THE FACTS PICTORIALLY ILLUSTRATED ON THE OPPOSITE 
 
 PAGE. 
 
 Anamnia. 
 
 Amniota. 
 
 s-l.ll 
 3* 
 
 Develops in all Anamnia, but 
 rarely persists as a permanent 
 excretory organ. 
 
 Still develops in the Amniota, 
 but as an excretory organ under- 
 goes entire degeneration in the 
 embryo : it may take part in the 
 formation of the suprarenal 
 body (?) 
 
 In Elasmobranchii, appears to Probably persists as the meso- 
 give origin by subdivision to nephric (Wolffian) duct, and con- 
 both mesonephric (Wolffian) ; tributes in some to the forma- 
 and Miillerian ducts. In Am- i tion of the Miillerian duct, 
 phibia, becomes converted into 
 the mesonephric duct. Its fate in 
 other Anamnia is not yet fully 
 investigated. 
 
 Functions in all Anamnia as a 
 urinary gland. In Elasmo- 
 branchs, Amphibians, and one 
 or two higher Fishes, its anterior 
 portion becomes related to the 
 male genital apparatus, the 
 posterior portion persisting as a 
 permanent kidney. 
 
 Loses its renal function in all 
 Amniota (as a rule in the em- 
 bryo), and becomes vestigial, 
 except so far as it becomes an 
 accessory portion of the repro- 
 ductive apparatus in the male 
 and enters into the formation of 
 the suprarenal body (?) 
 
344 
 
 2 
 
 f 
 
 COMPARATIVE ANATOMY 
 
 TABULATED RESUME ( Continued). 
 
 Anamnia. Amniota. 
 
 The proximal portion becomes 
 in most cases (except in Cyclo- 
 stomes and Teleosts) related to 
 the testis and functional in the 
 transmission of the semen, the 
 distal functioning as a kidney. 
 
 Persists as the kidney. 
 
 The proximal end becomes the 
 rete and vasa efferentia testis, 
 the caput epididymis, and per- 
 haps also the stalked hydatid 
 of Morgagni : the distal end be- 
 comes the paradidymis (Giralde's 
 organ). 
 
 The greater part of the proxi- 
 mal portion becomes the par- 
 ovarium, the distal the paroo- 
 phoron. 
 
 3 ! 0) 
 P. ' ** 
 
 Functions in most higher 
 Fishes merely as the urinary 
 duct. 
 
 In Elasmobranchs, Amphi- 
 bians, and some Ganoids, serves 
 as the urinogenital duct. 
 
 Functions exclusively as the | 
 duct of the mesonephros, i.e., 
 the urinary duct. | 
 
 The proximal portion becomes, 
 the corpus and cauda epidymis 
 and the distal the seminal duct 
 (vas deferens). 
 
 The greater part, as a rule, 
 degenerates; the proximal por- 
 tion maybe retained in a vestigial 
 form in the region of the par- 
 ovarium. In certain cases it 
 may persist, as a whole, as 
 Gartner's canal. The distal end 
 becomes the organ of Weber. 
 
 & ! 
 
 In Elasmobranchs it degene- 
 rates in post-embryonic life, 
 vestiges of its proximal portion 
 being retained. Its existence 
 in most other Fishes is doubt- j 
 ful. In Dipnoi and Amphibia it ' 
 is retained, at any rate for some j 
 time, for its whole length, in a 
 functionless and often but little ! 
 degenerate condition. 
 
 When present, becomes 
 whole genital duct. 
 
 the 
 
 The proximal portion becomes 
 the unstalked hydatid of Mor- 
 gagni, the distal, in some Mam- 
 mals, the so-called ' ' uterus 
 masculinus." In exceptional 
 cases the whole is retained as 
 Rathke's duct. In Sauropsida 
 the distal part usually dis- 
 appears. 
 
 Becomes 
 
 duct. 
 
 the whole genital 
 
 ! 
 J 
 
 *3 I Probably unrepresented (comp 
 lp.352). 
 
 Appears to arise in part (ure- 
 ter) from the distal end of the 
 mesonephric duct, and in part 
 (secreting elements) as a caudal 
 extension of the mesonephros. 
 
URINOGENITAL ORGAN'S 
 
 345 
 
 The pronephros itself has only a transitory function as an 
 excretory organ. Its duct, however, always persists, and usually 
 undergoes important modifications, which are closely connected 
 with the appearance of a second and more extensive series of 
 
 cp f*3Ts 
 
 5 jli{4 
 
 > K Jill g J' 
 
 S I (D-S Q 
 
 
 V^ EH ^i^aj* 
 
 r|b JS ^-5^ 
 
 ^g 1 1 IJ-g.J 
 
 ^^ " K 'S sJ 
 
 * "^ O O ^ T^ ^ r^ 
 
 B^ ^ 
 
 > H es 
 cc H 
 
 3 S 
 
 <; M o 
 
 , 
 
 ^ g 
 
 s ^ 
 
 
 
 se J-g 
 o g 
 ^ 
 
 
 lll 
 
 excretory segmental tubules, which appear later, mainly posteriorly 
 to the pronephros, and constitute the mesonephros or mid- 
 kidney ; the pronephric duct now serves as a mesonephric duct. 
 
 The mesonephros, often known as the Wolffian body (Figs. 
 273, 274, 277, B), is sometimes regarded as corresponding simply to 
 
346 
 
 COMPARATIVE ANATOMY 
 
 a " later generation " of pronephric tubules. It appears more 
 probable, however, that this organ originates independently from 
 a part of the mesoblastic somites situated more dorsally than that 
 which gives rise to the pronephric tubules. Primitively, the 
 mesonephros is strictly metameric, owing to the fact that each of 
 its tubules corresponds to the primary channel connecting the 
 cavity of a somite with the unsegmented coelpme (Fig. 274). The 
 loss of connection between these two sections of the primary 
 ccelome results in a series of segmental nephridia, each of which 
 
 opens into the body-cavity by a nephros- 
 tome, while at its other, or blind end, it 
 comes into connection with the prone- 
 phric duct or mesonephric duct as it 
 must now be called (Fig. 275). The 
 glomus of the pronephros is continued 
 backwards, and in the region of the 
 mesonephros breaks up into portions, or 
 glomeruli, each of which is situated in a 
 small cavity constricted off from the 
 coelome and opening into a mesonephric 
 tubule, forming what is known as a 
 Malpigliian capsule (Figs. 274, 275). 
 
 Each mesonephric tubule, then, in 
 its primitive form, is made up of the 
 following portions (Fig. 275) : (1) a 
 funnel-shaped ciliated aperture, commu- 
 nicating with the body-cavity (nephro- 
 stome, or peritoneal funnel) ; (2) a 
 rounded mass of capillaries (glomerulus\ 
 which is situated within a cavity (Mal- 
 piyliian capsule) derived from the 
 ccelome ; and (3) a coiled glandular 
 tubule^ opening into a collecting (me- 
 sonephric) duct. Thus the mesonephros, 
 as well as the pronephros, besides its 
 main function of excreting waste pro- 
 ducts by means of the epithelial cells 
 lining the tubules, serves also to conduct water derived from the 
 blood in the glomeruli, and peritoneal fluid, from the body. 
 
 The mesonephros is of greatest importance in the Anamnia : in 
 many Fishes it serves exclusively as a urinary organ, but in 
 Elasinobranchs and higher forms it also takes on certain relations 
 to the generative apparatus, giving rise to the rete and vasa 
 efferentia of the testis, as well as to the parorcliis or epididymis (p. 
 350), and, in Amniota, to other more or less rudimentary organs 
 of secondary importance (compare Fig. 273). Nevertheless, it may 
 still serve as the permanent urinary organ (Elasmobranchs, Am- 
 phibians), or may more or less entirely disappear as such (Amniota) ; 
 in the latter case, a third series of tubules is formed, giving rise 
 
 FIG. 275. DIAGRAM OF THE 
 MESONEPHRIC TUBULES, 
 
 SHOWING THEIR (SECOND- 
 ARY) CONNECTION WITH THE 
 MESONEPHRIC DUCT (SO). 
 
 The two anterior tubules are 
 already connected with the 
 duct, while the two posterior 
 have not yet reached so far. 
 
 ST, nephrostome; M, Mal- 
 pighian capsule with glome- 
 rulus ; DS, coiled glandular 
 tubule ; ES, terminal por- 
 tion of latter. 
 
URINOGENITAL ORGANS 347 
 
 to a metanephros, or hind-kidney, with which is connected a 
 metanephric duct or ureter. 
 
 The metanephros corresponds to a ]ater developed posterior 
 section of the mesonephros. Each metanephric duct apparently 
 arises as a hollow outgrowth from the posterior end of the meso- 
 nephric duct, where the latter opens into the cloaca. It gradually 
 extends forwards, and conies into connection with a series of 
 tubules developed as buds from the hinder end of the mesoue- 
 phros and provided with ccelomic Malpighian capsules and with 
 glomeruli, but not with nephrostomes. The posterior end of the 
 ureter soon loses its connection with the mesonephric duct, and 
 opens independently either into the cloaca or into a urinary bladder 
 (Figs. 294297). 
 
 THE MALE AND FEMALE GENERATIVE DUCTS. 
 
 In the Elasmobranchii, Amphibia, and Amniota, two canals are 
 formed in connection with the primary excretory apparatus : one 
 of these is known as the secondary mesonephric or Wolffian duct 
 which in male Elasmobranchii and Amniota functions as a seminal 
 duct or va-s deferens and in male Amphibia as a urinogenital duct, and 
 the other as the Mullerian duct which opens anteriorly into the 
 coelome and serves in the female as an oviduct (Figs. 278, 279). 
 The Wolffian duct becomes rudimentary in the female except in 
 Amphibians, in which it still serves as a urinary duct (Fig. 279) 
 and the Mullerian duct remains in a more or less rudimentary 
 condition in the male. These two ducts in some cases (Elasmo- 
 branchs) arise by a splitting of the primary mesonephric duct 
 into two (Fig. 278), but more usually the Mullerian duct arises 
 independently from the ccelomic epithelium. All the urinogenital 
 ducts are lined by a mucous membrane, external to which are 
 muscular and connective tissue layers. (For the relations of the 
 urinary and generative ducts in other Fishes and in Dipnoans 
 see pp. 360-363.) 
 
 THE GONADS (" GENERATIVE GLANDS "). 
 
 The sexual cells, which give rise to the ova and spermatozoa 
 originate from the germinal epithelium, which corresponds to 
 a differentiation of part of the ccelomic or peritoneal epithelium on 
 the dorsal side of the body-cavity on either side of the mesentery, 
 and into which the adjacent mesoblastic stroma penetrates ; thus 
 a pair of gonads or " sexual glands " is formed (Fig. 274). 
 
 Primitively the gonads were arranged segmentally, and extended through- 
 out a greater number of body segments (compare Amphioxus, p. 359). 
 
 The primitive germinal cells are at first entirely undifferen- 
 tiated, but in the course of development a differentiation takes 
 place, resulting in the formation of a male or a female gonad, i.e., 
 a testis or an ovary. 
 
348 
 
 COMPARATIVE ANATOMY 
 
 The mode of development of the ova and spermatozoa is briefly as 
 follows : 
 
 Ova. The cells of the germinal epithelium grow inwards amongst the 
 stroma of the ovary in the form of clustered masses : some of these increase 
 in size more than the others, and give rise to the ow, while the smaller cells 
 form an investment of follicle round them, and serve as nutritive material. 
 The investing cells multiply, and in Mammals a cavity containing a fluid 
 is formed in the middle of each follicle (Fig. 276) : the main mass of the 
 follicular cells which enclose the ovum project, as the discus proligerus, into 
 the cavity of the follicle. When ripe, the ovum, surrounded by a vitelline 
 membrane, comes to the surface of the ovary and breaks through into 
 the abdominal cavity ; it then passes into the coelomic aperture of the 
 oviduct. A certain amount of blood is poured out through the broken ends 
 of the vessels in the stroma of the ovary into the cavity of the follicle in 
 which the ovum lay : this " wound " then closes up, and its contents undergo 
 fatty degeneration, giving rise to a body of yellow colour, known as the 
 corpus luteum. 
 
 Spermatozoa. As in the case of the female, primitive germinal cells can 
 be at first distinguished in the development of the male generative elements. 
 These give rise to a series of seminal tubules (Fig. 300), containing larger and 
 smaller cells ; the former undergo division to form the sperm-cells or 
 
 g 
 
 FIG. 276. SECTION THROUGH A PORTION OF THE OVARY OF A MAMMAL, SHOWING 
 
 THE MODE OF DEVELOPMENT OF THE GRAAFIAN FOLLICLES. 
 
 KE, germinal epithelium, ingrowths from which extend into the stroma of the 
 ovary to form the ovarian tubes (PS) : the stroma is penetrated by vessels 
 (g.g) ; U, U, primitive ova ; S, cavity between the follicular epithelium 
 (tunica granulosa, M g) and the primitive ova ; Lf, liquor folliculi ; D, discus 
 proligerus ; Ei, ripe ovum, with its germinal vesicle (K) and germinal spot ; 
 Mp, zona pellucida, showing radiated structure ; Tf, theca folliculi. 
 
 spermatozoa. The nucleus gives rise to the so-called "head" of the sperma- 
 tozoon, while the surrounding protoplasm becomes differentiated to form the 
 motile "tail," which serves as an organ of propulsion, the "neck" 
 (Mittelstiick) arising from the centrosome of the cell (p. 3). 
 
UPvINOGENITAL ORGANS 
 
 349 
 
 1>. SPECIAL PAKT. 
 URINARY ORGANS. 
 
 In Amphioxus a series (90 or more) of independent segmental 
 tubules are present on either side in the reduced section of the 
 ccelome situated on the dorsal side of the pharynx (" dorsal 
 
 B 
 
 FIG. 277. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A, AMPHIOXUS, IN 
 THE BRANCHIAL REGION, AND B, AN ELASMOBRANCH EMBRYO, BASED ON 
 BOVERI'S FIGURES. 
 
 In A, the section passes through a branchial cleft on the right side, and shows a 
 transverse section of the anterior limb of a nephridium (X) ; on the left, a 
 nephridium (X) is indicated showing its communication with the ccelome (B) 
 and with the atrial chamber (C). A, genital section of ccelome (an ovary is 
 indicated on the right side) ; 7), section of the ccelome which extends down 
 the branchial bars ; F, ventral aorta. 
 
 la B, the section represents the pronephric region on the left, and the meso- 
 nephric region on the right. A, rudiment of a mesonephric tubule, the 
 blind end of which subsequently comes to open into the pro- (or meso-) 
 nephric duct (C) as indicated by the dotted lines on the right. B, nephro- 
 stome ; D, ccelome : F, subintestinal vein. 
 
 In both figures, E, lumen of gut ; G, aorta ; H, portion of commissural Vessel 
 which comes into relation with the excretory system. 
 
 coelomic canals"). Each of these tubules comes into close relation 
 with a branchial blood-vessel, possesses a varied number of lateral 
 branches, and opens on the one hand into the ccelome by several 
 ciliated funnels or nephrostomes ; and on the other by a single 
 aperture into the atrial or peribranchial chamber (p. 275), which 
 
350 COMPARATIVE ANATOMY 
 
 thus also serves as an excretory duct (Figs. 219 and 272, A). 
 The segmental arrangement of the tubules in the adult corre- 
 sponds to that of the branchial apparatus, and not to that of the 
 myotomes. No nephridia are present posteriorly to the pharynx, 
 and it is possible that the excretory system of Amphioxus may be- 
 to a certain extent comparable to an early stage of the pronephros- 
 of the Craniata. 
 
 In Cyclostomes the pronephros persists beyond the larva] 
 period, and for some time at any rate, functions as the sole excre- 
 tory organ : it possesses three or four nephrostomes. In Petro- 
 myzon it is soon replaced by a mesonephros, and the pronephros 
 then becomes rudimentary : between the two a fat-body is situated. 
 In Myxine it is uncertain whether the whole kidney,, or only its 
 anterior part, represents the pronephros. The kidney does not 
 come into relation with the generative organs, and its duct, which 
 opens on either side into the urinogenital sinus, probably represents 
 the unaltered pronephric duct. 
 
 In the Teleostei the pronephros has, in the majority of cases, 1 
 only a temporary significance, and the mesonephros constitutes 
 the excretory organ of the adult : it consists of a narrow 
 band varying in size and diameter in different regions, situated 
 on the dorsal side of the body-cavity, between the vertebral 
 column and the air-bladder. Secondary fusions between the 
 organ of either side often occur. The urinary duct in both groups 
 probably represents the pronephric duct, and may lie more or less 
 freely, or be embedded in the substance of the kidney. Posteriorly 
 the two ducts usually fuse together and become expanded to form 
 a kind of urinary bladder (compare Figs. 286 and 287) which has 
 nothing to do with the allantoic bladder of Amphibia and Am- 
 niota. The bladder usually opens behind the anus either inde- 
 pendently or together with the genital ducts by a simple pore, or 
 on the summit of a urinogenital papilla. 
 
 Thus a differentiation of the pronephric (or primary mesonephric) 
 duct into a Wolfnari and a Mtillerian duct is not known to occur in 
 Teleostei, nor does the mesonephros come into connection with 
 the gonads; in Elasmobranchii, in which the pronephros is 
 more rudimentary, this differentiation takes place (p. 346), and 
 at the same time a distinction between an anterior and a 
 posterior section of the mesonephros may be observed (compare 
 Figs. 278, 289, and 290). In the male, the former (paroreliis or 
 epididymis) comes into connection with the testis by means 
 of small ducts, the vasa efferentia, and its tubules open directly 
 into the Wolfnan duct, which thus functions as a vas deferens 
 only ; while the latter, which persists as the permanent kidney, 
 empties its secretion by means of separate urinary ducts into 
 the base of the Wolffian duct. In the female the Wolfnan 
 
 1 It is said to persist in Fierasfer, Lophius, Dactylopterus, Orthagoriscus mola. 
 Mora mediterranea, and the Afacrurida?. 
 
URINOGENITAL ORGANS 
 
 351 
 
 FIG. 278A. DIAGRAM or THE PRIMITIVE CONDITION OF THE KIDNEY IN AN 
 ELASMOBRANCII EMBRYO. (After Balfour.) 
 
 pd, pronephric duct : it opens at o into the body-cavity, and its other extremity 
 communicates with the cloaca ; x, line along which the division appears which 
 separates the pronephric duct into the Wolffian duct above, and the Miillerian 
 duct below ; s.t, nephridial tubes : they open at one end in the body-cavity, 
 and at the other into the Wolffian duct. 
 
 s.t- 
 
 FIG. 278s. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN 
 AX ADULT FEMALE ELASMOBRANCH. (After Balfour.) 
 
 m.d, Miillerian duct ; w.d, Wolffian duct ; d, urinary duct ; s.t, nephridial tubes : 
 five of them are represented with openings into the body-cavity : the posterior 
 nephridial tubes form the functional kidney ; ov, ovary. 
 
 S.t 
 
 FIG. 278c. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN 
 AN ADULT MALE ELASMOBRANCH. (After Balfour. ) 
 
 m.d, rudiments of Miillerian duct ; w.d, Wolffian duct, marked vd in front, and 
 serving as vas deferens ; s.t, nephridial tubes : two of them are represented 
 with openings into the body-cavity : the anterior tubules give rise to the 
 parorchis or epididymis and the posterior ones to the functional kidney ; d, 
 urinary duct ; t, testis ; nt, canal at the base of the testis ; VE, vasa 
 efferent ia ; Ic, longitudinal canal of the Wolffian body. 
 
352 COMPARATIVE ANATOMY 
 
 duct is rudimentary, and the ova pass to the exterior by means of 
 the Mullerian duct. 
 
 This special differentiation of the hinder part of the meso- 
 nephros, and the formation of special ducts in connection with it, 
 seems to foreshadow the condition which occurs in the Amniota 
 (pp. 346 and 356). 
 
 The anterior (sexual) part of the kidney is usually broader than the 
 posterior (renal) part. The outer border is usually notched, and this, 
 together with the arrangement of the nephrostomes in the embryo, points 
 to the original segmental arrangement of the organ. The segmental char- 
 acter, however, disappears later on ; in the adult the nephrostomes are 
 much less numerous than the vertebras of this region, but their number 
 and size vary much in different genera and even in individuals, and they 
 apparently do not persist in all. 
 
 The morphology of the kidneys and renal ducts in Ganoids (Figs. 
 286 and 287) requires further investigation. They seem on the 
 whole to resemble those of Teleosts, though in the Sturgeon they 
 apparently show points of similarity to those of Elasmobranchs. 
 As in the Teleostei, a well-developed pronephros is present in 
 the larva, and the kidney duct probably represents the pronephric 
 duct. 
 
 In many Fishes the kidneys extend far back into the root of 
 the tail. 
 
 A close examination of the organ, which appears to the naked eye as the 
 kidney in Teleosts and Ganoids, shows that a larger or smaller portion of it 
 more particularly the anterior part consists of an adenoid or lymphoid 
 substance. 
 
 In the Dipnoi the kidneys also undoubtedly correspond to 
 the mesonephros. They are relatively longer in Protopterus 
 (Fig. 288) than in Ceratodus, extending through a considerable 
 portion of the body-cavity : as in Elasmobranchs, a narrower 
 anterior can be distinguished from a broader posterior part, and 
 the whole is largely invested by lymphoid and adipose tissue. 
 Nephroatomes are wanting. Until their development is known, 
 it is uncertain to what extent the renal ducts correspond to the 
 primary mesonephric ducts : each opens into the cloaca inde- 
 pendently, behind the genital aperture. The cloacal caecum (p. 262) 
 probably functions as a urinary bladder. 
 
 Amphibia. The pronephros is well developed in the larva, 
 and is particularly large in the Gymnophiona, in which as many as 
 12 or 13 nephrostomes may be present. 
 
 In adults, the most primitive condition is met with in the 
 Gymnophiona, in which the kidney (mesonephros) consists of long, 
 narrow, varicose bands, usually extending from the heart to the 
 anterior part of the cloaca, which latter is often much elongated. 
 In the embryo they consist of definite masses, which are arranged 
 metamerically, and in each of them a glomerulus, a nephrostome, 
 and an excretory duct can be distinguished (Fig. 291). This con- 
 dition sometimes persists in the anterior portion of the kidney, 
 
URINOGENITAL ORGANS 
 
 353 
 
 but, owing to secondary processes of growth, as many as twenty 
 nephrostomes are later on met with in a single body- segment. The 
 number of nephrostomes in the entire kidney may amount to a 
 thousand or more. 
 
 As regards the urinary duct and the relations of the entire 
 
 A 
 
 FIG. 279. DIAGRAM OF THE URINOGENITAL SYSTEM OF (A) A MALE AND (B) A 
 FEMALE URODELE ; FOUNDED ON A PREPARATION OF Triton tceniatus. 
 (After J. W. Spengel. ) 
 
 Ho, testis ; Ve, Ve, vasa efFereiitia of testis, which open into the longitudinal canal 
 of the mesonephros, f ; a, collecting tubes of the mesonephros, which open into 
 the Wolffian (urinogenital) duct (Ig, Ig) ; in the female the latter serves 
 simply as the urinary duct ( Ur), and the system of the vasa efferentia (testicu- 
 lar network) is rudimentary ; mg, mg l (Od), Miillerian duct ; Of, coelomic 
 aperture of latter in the female ; Ov, ovary ; (?JV, anterior sexual portion of 
 kidney (parorchis of the male) ; N, posterior non-sexual portion of kidney. 
 
 renal apparatus to the generative organs, the Gymnophiona in 
 all essential points resemble other Amphibia. 
 
 The kidneys of Urodela and Anura are situated in the usual 
 position on the dorsal side of the body- cavity ; in the former they are 
 band-like and more extended longitudinally than in the latter^ 
 
 A A 
 
354 
 
 COMPARATIVE ANATOMY 
 
 CvAo 
 
 in which they are shorter and more compact, and are confined 
 to the middle portion of the ccelome. 
 
 In Urodeles they always consist of a narrow anterior, and a 
 broader and more compact posterior portion. The latter, as in 
 Elasmobranchs, gives rise to the functional kidney (Fig. 279), 
 while the former becomes connected in the male with the 
 generative organs. Delicate vasa efferentia, developed from the 
 mesonephros, pass out from the testis (Figs. 279, 280, 
 292) into the substance of the kidney, and there open into the 
 renal tubules; they may either enter the kidney direct, or 
 
 else open first into a longitudinal 
 collecting duct, from which fine 
 canals pass to the tubules. Thus 
 the seminal fluid passes through 
 the nephridia as well as through 
 the Wolfnan duct, which serves as 
 a urinogenital duct. 
 
 In Urodela and Anura of both 
 sexes the Wolffian duct nearly 
 always opens separately on either 
 side into the cloaca, receiving first, 
 in Urodeles, a number of ducts 
 from the posterior part of the 
 kidney (compare Elasmobranchs, 
 p. 350). In Anura the Wolffian 
 ducts pass some distance indepen- 
 dently along the body-cavity, in 
 correspondence with the position 
 of the kidneys, and a seminal 
 vesicle opens into each (Fig. 281). 
 The urinary (allantoic) bladder 
 (see p. 259) opens into the cloaca 
 ventrally, opposite to the urino- 
 genital apertures. In its simplest 
 form it is finger-shaped (e.g., Siren, 
 Proteus), but it usually becomes 
 swollen distally and is often bi- 
 lobed : in Alytes and Bombinator 
 it forms a double sac. 
 
 Slight indications of a seg- 
 mental arrangement are found only 
 in the anterior sexual portion of 
 
 the kidney of Urodeles ; in the posterior part, and in the entire 
 kidney of Anura, all traces of segmentation have disappeared. In 
 both cases, however, the nephrostomes remain throughout life 
 in great numbers on the ventral surface of the kidney, which 
 is covered over by the peritoneum (Fig. 281). 
 
 The nephrostomes are connected with the urinary tubules in larval Anura, 
 but later on they become separated from them, and open into the renal 
 
 FIG. 280. MALE URINOGENITAL 
 ORGANS OF Rana esculenta. 
 
 Ur, Ur, Urinogenital (Wolffian) 
 ducts, which appear on the lateral 
 surface of the kidneys at f; 8, 
 S', their apertures into the cloaca 
 (Cl) ; Ho, Ho, testes ; FK, FK, 
 corpora adiposa ; Cv, postcaval 
 vein ; Ao, aorta ; Vr, revehent 
 renal veins. 
 
URIXOGEXITAL ORGANS 
 
 355 
 
 FIG. 28 L KIDNEY OF Discoglossu* pictus. From the ventral surface, showing 
 the nephrostomes (ST). (After J. W. Spengel). 
 
 Ur, urinogenital duct, enlarging at Ur 1 to form a seminal vesicle. 
 
 veins. In consequence of this change of function, for such it must be 
 considered, the body-cavity of adult Anura forms a closed lymph-sinus, as in 
 the Amniota ; the peritoneal fluid, which in the larva was carried to the 
 exterior and lost, is in the adult poured into the general circulation, like the 
 rest of the lymph. 
 
 A A 2 
 
356 
 
 COMPARATIVE ANATOMY 
 
 Reptiles and Birds. In the Sauropsida, as in the Mammalia, 
 the mesonephros, so far as it is retained in the adult, is entirely 
 separate from the functional excretory apparatus ; this consists 
 of a metanephros, entirely wanting in nephrostomes (compare 
 p. 346 and Fig. 273). 
 
 The metanephros never extends so far along the body-cavity 
 as does the mesonephros; as a rule it has the form of a small 
 compact or lobulated organ, usually situated in the posterior 
 half of the body-cavity, or even entirely confined to the pelvic 
 region : it has the latter position, for instance, in most Reptiles 
 
 FIG. 282. EXCRETORY APPARATUS OF Monitor indicus. 
 
 The right kidney is shown in its natural position, while the left is turned on its 
 longitudinal axis, so that the ureter and the collecting tubes are visible. The 
 urinary bladder is not represented. 
 
 N, N, kidneys ; SG, collecting tubes which open into the ureter (Ur, Ur 2 ) ; CTr 1 , 
 aperture of ureter into the cloaca. 
 
 (Figs. 282, 294 and 295) and all Birds (Fig. 283). The posterior 
 end of the kidney, which is generally narrower than the rest, 
 may even extend under the root of the tail, as in Lacerta, in 
 which region there is a fusion of the organ of either side. 
 
 Thus, according to the position of the kidneys, the ureters 
 (metanephric ducts) either do not extend freely along the 
 body-cavity, or they may have a longer or shorter free course. 
 The latter is the case, for instance, in Crocodiles, and more 
 especially in Birds (Fig. 283) : in the latter the kidneys 
 are closely embedded within the pelvis, and their ventral 
 flattened surface, which is usually divided into three lobes, is 
 
URINOGENITAL ORGANS 
 
 357 
 
 often penetrated by deep furrows and clefts in which the veins 
 lie embedded ; posteriorly they may fuse together in the middle 
 line, as in Lizards. 
 
 There is not always a perfect symmetry between the organ 
 of either side, and this is most marked in Snakes, in which the 
 
 An 
 
 FIG. 283. MALE URINOGENITAL APPARATUS OF HERON (Ardea cinerea). 
 
 JV, kidneys ; Ur, ureter, opening into the cloaca (O) at Sr ; Ho, testis ; Ep, epi- 
 didymis ; Vd, vas deferens, which opens at Vd l op a papilla in the cloaca : 
 V, V, furrows on the ventral surface of the kidney in which veins lie em- 
 bedded ; Ao, aorta ; BF, bursa Fabricii, which opens into the cloaca at BF 1 . 
 
 greatly tabulated kidneys, like those of limbless Lizards, are 
 elongated, narrow, and band-like, in correspondence with the 
 form of the body. 
 
 A urinary (allantoic) "bladder arising from the ventral wall of 
 the cloaca, is present in Lizards and Chelonians ; it is more or less 
 bilobed. A bladder is wanting in Snakes, Crocodiles, and Birds. 
 
358 
 
 COMPARATIVE ANATOMY 
 
 FIG. 284. DIAGRAMMATIC LONGI- 
 TUDINAL SECTION THROUGH THE 
 KIDNEY OF A MAMMAL. 
 
 Mammals. The definitive kidneys (metanephros) of Mammals 
 are proportionately small, and lie on the quadratus lumborum 
 muscle and ribs. They usually possess a convex outer, and a 
 concave inner border; the latter is called the hilum, and at this 
 point the ureters arise and the blood-vessels enter. The expanded 
 proximal portion of the ureter is divided up to form one 
 or more calyces (Fig. 284), into which small papilliform processes 
 
 of the pyramids (see below) project; 
 on the summits of these the urinary 
 tubules open in varying number. 
 The calyces are continuous with a 
 large cavity in the widened portion 
 of the ureter called the pelvis, and 
 from this the ureter (metanephric 
 duct) passes freely backwards for 
 some distance to open into the 
 bladder (except in Monotremes, Fig. 
 296) on its dorsal side, sometimes- 
 nearer the apex, sometimes towards 
 the fundus. The bladder communi- 
 cates with the urinogenital canal or 
 urethra. 
 
 The kidney is greatly lobulated 
 in the embryo; this condition may 
 remain throughout life, or the lobes 
 may become more or less completely 
 united (Fig. 285). In the latter case 
 the original division into lobes may 
 
 still be recognised more or less plainly internally. A section of 
 the kidney shows an inner layer, the medullary substance, arranged 
 in the form of wedges the urinary pyramids, and an outer layer, 
 or cortical substance, extending as the columns of Bertini between 
 the pyramids (Fig. 284). The pyramids correspond roughly to the 
 embryonic lobes of the kidney, though several lobes may fuse 
 together in one pyramid. 
 
 The glomeruli as well as the coiled portions of the tubules, sur- 
 rounded by a network of blood-capillaries, lie in the cortical 
 substance, while the straight portions of the tubules extend through 
 the pyramids, where they gradually anastomose to form larger 
 collecting tubes. 
 
 The greater part of the urinary bladder does not corre- 
 spond with the proximal end of the allantois, but to a special 
 differentiation of the cloaca, which becomes divided into a 
 dorsal and a ventral portion by the formation of a horizontal 
 septum. The ventral portion gives rise to the bladder, which 
 is continuous distally with the stalk of the allantois (urachus, 
 see p. 340), from which the median ligament of the bladder 
 is formed. In Monotremes and nearly all Marsupials (see 
 
 R, H, cortical substance ; M, M, 
 medullary substance arranged in 
 pyramids (Pr) ; between the 
 latter the cortical substance ex- 
 tends in the form of the columns 
 of Bertini (B, B) ; Ca, calyces ; 
 Pe, pelvis ; Ur, ureter. 
 
URINOGENITAL ORGANS 
 
 359 
 
 p. 338) the whole allantois takes part in the formation of the 
 bladder. 
 
 Z2K 
 
 FIG. 285. A, RIGHT KIDNEY OF A DEER; B, KIDNEYS (X) AND SUPRARENAL 
 
 BODIES (JN T .A T ) OF THE HUMAN EMBRYO. (Both from the ventral side.) 
 
 Ur, ureters. 
 
 GENERATIVE ORGANS. 
 
 In Amphioxus the gonads are developed in a part of the 
 reduced coelome situated on either side of the pharynx and 
 intestine (Fig. 277, A) between the outer body-wall and the atrial 
 cavity. They have a marked segmental arrangement, and each 
 portion sheds its products independently into the atrial cavity, 
 whence they pass out through the atrial pore (compare p. 275 and 
 Fig. 219). 
 
 In Cyclostomes also, generative ducts are wanting ; the sper- 
 matozoa or ova are shed directly into the body-cavity, and pass 
 through the genital pores (p. 298) into the urinogenital sinus, and 
 so to the exterior. The gonad is a long unpaired organ suspended, 
 as in other Vertebrates, to the dorsal wall of the body-cavity by 
 a fold of peritoneum, the mesorchinm or mesoarium, as the case 
 may be. 
 
 In Fishes the gonads are only exceptionally unpaired, and 
 even then, this is only a secondary condition, due to the fusion 
 of the two organs or to the reduction of that of one side ; as in all 
 
360 COMPARATIVE ANATOMY 
 
 other Vertebrates, they are originally paired. There is usually a 
 want of symmetry observable between the organ of the right and 
 left sides. 
 
 The testes and ovaries of Teleostei closely correspond with one 
 another as regards position and the arrangement of their ducts. 
 Dorsal and ventral folds of the peritoneum are developed in con- 
 nection with the elongated ovary, and these in most cases meet 
 along its outer side, so as to enclose a portion of the ccelome and 
 thus convert the ovary into a hollow sac, blind anteriorly, 
 on the inner folded walls of which the ova arise ; this sac is 
 continued backwards to form the oviduct (compare Fig. 286). The 
 latter, which is generally short, as a rule fuses with its fellow to 
 form an unpaired canal; this opens either by a genital pore 
 (p. 298) between the rectum and the urinary aperture on a level 
 with the integument, or on a papilla, which may become elongated 
 to form a tube or " ovipositor " ; or the ducts may communicate 
 with a urinogenital sinus. 
 
 The testis of Teleosts is elongated, and often lobulated in form. 
 Its duct has similar relations to those seen in the female. 
 
 Thus the ducts, both of the ovary and testis, correspond to folds 
 of the peritoneum enclosing a coelomic cavity continuous with that 
 of the gonads, and originate quite independently of the nephridial 
 system. The oviducts must therefore be distinguished from true 
 Miillerian ducts. 
 
 In some Teleosts the ovary is solid, and the ova are shed into 
 the body-cavity. In the Smelt (Osmerus) and in Mallotus the 
 oviducts (" peritoneal funnels ") have open ccelomic apertures close 
 to the ovaries into which the ova pass (compare Fig. 286, B) ; while 
 in other Salmonidse and in the MuraBnidse and Cobitis, for 
 instance, these peritoneal funnels are shorter, and may even be 
 absent, the ova then being shed into the urinogenital sinus 
 through a paired or single genital pore. It is uncertain whether 
 the latter is the primitive arrangement amongst Teleostei, or 
 whether the peritoneal funnels represent reduced oviducts. 
 
 Most Teleostei are oviparous, but viviparous forms occur (p. 336). The 
 male Stickeback builds a nest for the protection of the young formed of a 
 hardened secretion (muciii) of the kidney, which undergoes a change of 
 function at the breeding-season ; in Syngnathus and Hippocampus, the young- 
 are protected within a pouch on the abdomen of the male, and in the female 
 Solenostoma on a pouch between the ventral fins. Amongst Siluroids they 
 are carried within the pharynx in the male Arius, and the eggs are attached 
 to the soft ventral integument in the female Aspredo. 
 
 Amongst Ganoidei the female organs of Lepidosteus are formed 
 on the same type as those of the Teleostei. In Amia (Fig. 
 286, B) and Acipenser each oviduct opens by a funnel into the 
 <jcelome, but in all Ganoids the oviduct is probably comparable to 
 that of Teleosts, and not to a Miillerian duct. In the male 
 Lepidosteus a series of vasa efferentia pass out from the testis and 
 
UBINOGENITAL ORGANS 
 
 361 
 
 open into a longitudinal canal from which ducts pass into the 
 kidney, the duct of which therefore serves as a urinogenital duct 
 (Fig. 287). The latter dilates before uniting with its fellow to 
 open into the uriuogenital sinus. A somewhat similar arrange- 
 ment appears to occur in the male Sturgeon, in which representatives 
 of the oviducts of the female are present in the form of short ccelomic 
 funnels opening into the kidney-ducts. It is probable that the other 
 male Ganoids also resemble Lepidosteus in the structure of their 
 urinogenital organs, which, however, require further investigation. 
 In the Dipnoi (Fig. 288) the elongated gonads are invested 
 
 -vef 
 
 ovcL" 
 
 ovd. 
 
 ovd. 
 
 Z.c 
 
 FIG. 286. FIG. 287. 
 
 FIG. 286. FEMALE URINOGENITAL ORGANS OF A, LEPIDOSTEUS, AND B, AMIA. 
 (A, after Balfour and Parker ; B, after Huxley.) 
 
 , degenerate anterior portion of kidney ; bt, bladder ; hi, kidney ; ord, oviduct ; 
 ovd', aperture of oviduct into bladder ; crvd", peritoneal aperture ; ory, ovary ; 
 p, peritoneum ; u.g. aj), urinogenital aperture ; nr, kidney duct. 
 
 FIG. 287. MALE URINOGENITAL ORGANS or LEPIDOSTECS. (After Balfour and 
 
 Parker.) 
 
 bl, bladder ; Lc, longitudinal canal ; to, tesfcis ; u.g, ap, urinogenital aperture ; ur, 
 kidney duct ; v.ef t vasa efferentia. 
 
vd. 
 
 ANG 
 
 Poab 
 
 FIG. 288. URINOGENITAL ORGANS OF Protopterus annectens. A, FEMALE ; B, 
 YOUNG MALE. From the' ventral side. (After W. N. Parker. ) The peritoneum 
 is removed on the right side in A, and on the left in B. 
 
 In both figures : N,N~ l , kidneys ; Ost, ccelomic aperture of Miillerian duct ; (y, Ig, 
 lymphatic tissue in connection with the kidneys ; NG, kidney ducts : A NO, 
 apertures of kidney ducts into the cloaca (Cl) ; Pap, genital papilla in the 
 cloaca formed by the fusion of the base of the Miillerian ducts ; RD, cloacal 
 caecum ; Re, rectum ; Poab, abdominal pore ; Cp, postcaval vein, connected 
 by transverse anastomoses (GWJ.S-) with the left posterior cardinal vein ( V. card), 
 The veins from the gonads are also indicated. 
 
 In A : O V, Ov l , ovaries ; ovd, ovd', oviducts. 
 
 In B : Hod, testes, with their investment of lymphoid and adipose tissue (LG) ; 
 Hod.G, free portion of sperm-duct,uniting with the Miillerian duct (MG} 
 at *. Per, peritoneum. 
 
URINOGENITAL ORGANS 363 
 
 with lymphoid and adipose tissue, and are closely attached to the 
 outer border of the kidneys ; when ripe, they become greatly 
 enlarged, so as to embrace the gut ventrally. The oviducts, which 
 doubtless correspond to Mullerian ducts, are long and coiled, re- 
 sembling those of the Amphibia: posteriorly they unite before 
 opening into the cloaca, and each communicates anteriorly with the 
 body-cavity by a funnel-shaped aperture. The wall of the ovi- 
 duct secretes albumen round the eggs as they pass along it. 
 
 The manner in which the sperm is conducted to the exterior in 
 Ceratodus is not understood. In Protopterus (Fig. 288, B) the 
 seminal tubules of the testis open into a duct, embedded between 
 the lobes of the testis ventrally. Behind the testis the duct ex- 
 tends freely for a short distance, and then unites with the base of 
 the Mullerian duct, which, as in the female, joins with its fellow 
 to open into the cloaca on the summit of a papilla. The rest of 
 the Mullerian ducts, with their ostia, can still be recognised in 
 immature individuals. The sperm-duct of Protopterus is ap- 
 parently a structure sui generis ; like that of Teleosts, it is probably 
 developed quite independently of the nephridial system. 
 
 In the greater number of Elasmobranchs the ovaries are 
 paired, and the oviducts, like those of Dipnoans, are separate 
 from the ovaries, and correspond to Miillerian ducts. Their 
 anterior portion has a common opening into the body-cavity, 
 and further back each is provided with an oviducal or shell-gland. 
 The anterior part of the oviduct is always narrower and more 
 delicate than the posterior, which dilates to form a kind of uterus, 
 in which (when the Shark is viviparous) the embryo undergoes de- 
 velopment. Posteriorly, the oviducts open into the cloaca some- 
 what behind the aperture of the ureters either separately, or by 
 a common canal (Figs. 278, B, 289). 
 
 The oviducal gland secretes the horny material for the egg-case or 
 "purse," which is usually elongated and produced at its four angles into 
 longer or shorter tendril-like threads ; in Cestracion it has a spiral ridge. 
 The majority of Sharks are viviparous, and in these the egg-shell becomes 
 more or less reduced. In Mustelus antarcticus the uterus becomes divided 
 into several compartments, each containing an embryo surrounded by a 
 membrane apparently representing the horny egg-shell of other forms. 
 
 The testis of Elasmobranchs is paired and symmetrical, but the 
 two organs may become partially fused with one another. Vasa 
 efferentia connect each testis with the anterior end of the corre- 
 sponding mesonephros (parorchis), the Wolffian duct serving as a 
 vas deferens (p. 346), and giving rise to a dilated portion or 
 vesicula seminalis, as well as to a caacal sperm-sac where it com- 
 municates with the urinogenital sinus : the latter opens into the 
 cloaca on the apex of a papilla (Figs. 206, 278, c, 290). Vestiges of 
 the anterior end of the Miillerian ducts can be recognised even in 
 the adult as a short transverse tube with a central aperture,, 
 situated below the oesophagus anteriorly to the liver. 
 
364 
 
 COMPARATIVE ANATOMY 
 
 Fertilisation is internal in all Elasmobraiichs except Lsemargus, in which 
 generative ducts are said to be wanting. 
 
 FIG. 289. 
 
 FIG. 290. 
 
 FIGS. 289 and 290. FEMALE AND MALE URIXOGENITAL ORGANS or Raja batis. x ^. 
 (From T. J. Parker's Zootomy. ) 
 
 a.p, abdominal pore ; d, cloaca ; epd, left epididymis (the right is removed) ; 
 fl.t, anterior portion of oviducts (Miillerian duct); fl.t', common opening 
 of the two oviducts into the ccelome ; i.r, internal body ; k, kidney ; 
 mn.d, mesonephric (Wolffian) duct ; od.g, oviducal gland ; oes, oesophagus; 
 ov, right ovary (the left is removed) ; pn.d, remains of anterior part of 
 Miillerian duct ; ., sperm sac ; s.s', its opening into the urinogenital 
 inus ; t, left testis (the right is removed) ; u.b, dilated end of mesonephric 
 duct; ug.p, urinogenital papilla; tig.*., urinogenital sinus; u.p, urinary 
 papilla ; ur, kidney ducts ; tir', opening of kidney duet into the urino- 
 genital sinus ; lit, uterine portion of left oviduct (that on the right side 
 is removed) ; nt', its opening into the cloaca ; r.d, vas deferens ; v.s, 
 vesicula seminalis ; t'.-s-', its opening into the sperm-sac. 
 
 In female Holocephali the generative organs are essentially 
 similar to those of Elasmobranchs, but in the male they present 
 certain peculiarities. The unripe sperms pass into the immense 
 epididymis, in which they become ripe and invested by spermato- 
 
URINOGENITAL ORGANS 
 
 365 
 
 phores, which pass into a large vesicula seininalis at the base of 
 the vas deferens ; the Mlillerian ducts are complete in the male, 
 though small. 
 
 Herrncqyhroditism occurs amongst Fishes. In Myxine the posterior part of 
 the goiiad usually first gives rise to spermatozoa, and subsequently the anterior 
 part to ova, so that self -fertilisation is prevented. Serranus and Chryso- 
 phrys, again, are hermaphrodite, and hermaphroditism has been occasionally 
 observed in various other Fishes (e.g., Gadus morrhua, Scomber scomber, 
 Clupea, Harengus). 
 
 Amphibia. The paired and symmetrical gonads of Amphibians 
 are situated in about the middle of the coelome ; their 
 form is usually modified by the shape of the body. Thus in the 
 Gymnophiona the ovary has the form of a long and narrow band, 
 while the testis is made up of a long chain of small bodies united 
 together by a collecting duct (Fig. 291). Each individual portioa 
 
 JTo 
 
 -IfS 
 
 FIG. 291. DIAGRAM OF A PORTION OF THE MALE GENERATIVE APPARATUS OF 
 
 THE GYMNOPHIONA. 
 
 Ho, Ho, testis ; Sg, collecting duct of testis ; K, K, testicular capsules ; Q, Q 
 transverse canals connecting the collecting duct with the longitudinal canal 
 (L, L) ; Q 1 , Q 1 , second series of transverse canals ; M, M, Malpighian 
 capsules ; N, JV~, kidney ; ST, nephrostome ; S t convoluted portion of urinary 
 tubule ; HS, urinogenital duct. 
 
 of the testis of Caecilians is made up of a double row of rounded 
 capsules in which the sperm is formed, and from which it is passed 
 into a collecting duct, which perforates each portion of the organ. 
 A transverse canal is given off from the free portion of the collect- 
 ing duct lying between every pair of testis lobes ; this passes to- 
 
366 COMPARATIVE ANATOMY 
 
 wards the kidneys, and opens into a longitudinal canal. From the 
 latter the sperm passes through a second system of transverse canals 
 to the Malpighian capsules, and thence through the urinary tubules 
 into the urinogenital duct. 
 
 The male generative apparatus of all Urodela and certain 
 Anura (comp. p. 354) corresponds in the main with that seen in 
 Csecilians, except as regards the form of the gonads : thus the testis 
 is either pointed at one or both ends (Fig. 279), or more or less 
 round or oval (Figs. 280, 292). 
 
 In Rana, Bombinator, and Alytes the vasa effereiitia of the testis become 
 gradually separated from the kidney : that is, they either open directly into 
 the urinogenital duct, without becoming connected with the urinary tubules 
 
 m ^^m 
 
 Hr 
 
 FIG. 292. TESTIS AND ANTERIOR END OF KIDNEY OF Rana esculenta. 
 (Semidiagrammatic. ) 
 
 Ho, testis ; q, q, transverse canals of the testicular network, which give rise to 
 blind processes at f f ; L, longitudinal canal of the testicular network, from 
 which the interrenal network (C, C) arises ; N, kidney ; Ur, urinogenital 
 duct. 
 
 (Rana, Fig. 292), or the greater number of the posterior canals end 
 blindly, while only the anterior ones are directly connected with the 
 urinogenital duct (Bombinator). In Alytes the relations of the generative 
 ducts require further investigation. 
 
 Mulleriaii ducts are always present, but they always remain in a more or 
 less rudimentary condition in the male, and lie along the outer border of the 
 kidneys in a similar position to those of the female. They may or may not 
 be provided with a lumen and apertures of communication with the body- 
 cavity and cloaca. 
 
 Hermaphroditism occasionally occurs amongst the Anura : only one case 
 (Triton teeiiiatus) is known amongst Urodeles. A body attached to the 
 anterior end of the testis in various species of Toads ("Bidder's organ") is 
 apparently capable of giving rise to ova and spermatozoa. In the males of 
 Rana temporaria ova are at times developed, embedded within the sub- 
 
URINOGENITAL ORGANS 
 
 367 
 
 stance of the testis (hermaphrodite gland or ovotestis), and one testis may 
 even be replaced by a rudimentary ovary. In these cases, the Miillerian 
 duct may be as well developed as in the female. 
 
 The ovaries of Urodela are always formed on a common plan : 
 each consists of an elongated closed tube, with a continuous lumen. 
 In Anura, on the contrary, the ovarian sac (Fig. 293) is divided up 
 
 '-- ot 
 
 FIG. 293. URINOGENITAL ORGANS OF A FEMALE Rana escidenta. 
 
 Ov, left ovary (that of the right side is removed) ; Od, oviduct ; Ot, abdominal 
 aperture of oviduct ; Ut, the dilated posterior end of the oviduct ; P, 
 opening of latter into the cloaca ; ^V, kidneys ; S, S l , apertures of urinary 
 ducts into the cloaca, surrounded by longitudinal folds (*), which are 
 separated by a deep depression (t). 
 
 into a longitudinal row of numerous (3 to 20) separate pockets or 
 chambers. The oviducts open far forwards into the body-cavity by 
 funnel-shaped apertures; they take a tolerably straight course 
 along the outer borders of the kidneys to the cloaca in young 
 animals, but become greatly convoluted later. A short distance 
 from its termination each oviduct in Anurans becomes dilated to 
 form a thin- walled sac, and, after becoming again narrowed, usually 
 
368 COMPARATIVE ANATOMY 
 
 opens separately on a papilla on the dorsal wall of the cloaca. 
 Only in the genera Bufo and Alytes do the two oviducts fuse 
 together into a posterior unpaired canal. 
 
 In female Urodeles a number of tubes serving as a receptaculum seminis 
 may be present in connection with the cloaca, and the male is provided with 
 a cloacal gland which secretes spermatophores. 
 
 After receiving a gelatinous coating from the glands in the wall of the 
 middle part of the oviduct, the eggs of Anurans pass into the dilated portion 
 of the duct, and become united together into irregular masses (Frog) or chains 
 (Toad). 
 
 In Epicrium glutinosum (Gymnophioiia) the eggs are very similar to those 
 of Sauropsida : they are exceptionally large (9 mm. long), of an oval shape, 
 and possess a large yolk. They become coated after fertilisation with a 
 tough albumen in the oviduct, and this becomes drawn out at the poles into 
 chalazse, by means of which the eggs are connected together like the beads of 
 a necklace. The segmentation is meroblastic, and takes place in the oviduct ; 
 and the eggs are laid in the earth, the mother coiling herself round them. 
 
 Fat-bodies (corpora adiposa) are present in all Amphibia in 
 connection with the gonads : they are formed of adenoid substance, 
 fat, and leucocytes, and contain numerous blood-vessels. 
 
 The corpora adipqsa probably have an important physiological (nutritive) 
 relation to the gonads : Amphibians, after remaining for months without food, 
 throughout their winter sleep, are able as soon as spring arrives to give rise 
 to thousands of offspring. 
 
 Reptiles and Birds. The essential differences between the 
 urinogenital organs of the Anamnia and Amniota have already 
 been described (pp. 346, 356, and Fig. 273). a 
 
 In the Sauropsida, as in other Vertebrates, the form of the 
 gonads is influenced by that of the body : thus in Chelonians they 
 are broad, while in Snakes and snake- like Lizards they are more 
 elongated. In the latter cases, as well as in other Lizards, they 
 are asymmetrical, so that the organ of one side is situated more 
 or less in front of that of the other. More room is thus obtained 
 for the development of the ovaries ; and, in cases where the eggs 
 are very large, the organ of one side tends to disappear, as in 
 certain Elasmobranchs (e.g., Scyllium) : in Birds, for instance, the 
 left ovary only is completely developed and functional. 
 
 In Reptiles the ovaries are penetrated by a highly vascular 
 network of trabeculse, and in the lymph-cavities thus formed the 
 formation of ovarian follicles takes place. 
 
 The oviducts (Fig. 294) possess wide funnel-shaped abdominal 
 apertures, and are usually much folded transversely ; the right is 
 often longer than the left. Their walls are provided with numerous 
 muscular elements and glands for the formation of the albumen 
 and egg-shell, and they increase in size in the breeding-season. 
 In Birds the right oviduct, as well as the right ovary, becomes 
 more or less completely degenerated ; the left oviduct is considerably 
 coiled. 
 
URINOGENITAL ORGANS 
 
 369 
 
 Only slight remnants of the mesonephros and Wolffian duct persist 
 in female Reptiles, and these undergo fatty degeneration. They are asym- 
 metrical, and are arranged in a single row 011 either side, between the oviduct 
 
 Ot- 
 
 Od 1 
 
 FIG. 294. FEMALE URINOGENITAL APPARATUS OF Lacerta muralis. 
 
 N, kidneys ; C/r 1 , apertures of the ureters into the cloaca (Cl) ; B, urinary bladder ; 
 B 1 , neck of the latter (cut open) ; R, rectum ; A 1 , opening of rectum into the 
 cloaca ; Or, ovaries ; f, remains of mesonephros ; Od, oviducts, which open 
 into the cloaca at Od 1 ; Ot, abdominal openings of oviducts. 
 
 and vertebral column. The remains of the Wolffian duct are more 
 marked in female Snakes, Chelonians, and in Geckos than in the majority of 
 Lizards. 
 
 B B 
 
370 
 
 COMPARATR 7 E ANATOMY 
 
 The testes of Sauropsida correspond in position with the ovaries, 
 and, like them, increase in size in the breeding-season. They have 
 an oval, round, or pyriform shape (Figs. 283 and 295), and are made 
 
 up of greatly convoluted seminal 
 tubules, held together by fibrous 
 tissue. In Reptiles (e.g. Lacerta, 
 Anguis), " yellow bodies," which cor- 
 respond to suprarenals (p. 385), lie 
 along the outer border of the testis, 
 and at this point transverse canals 
 pass out from the testis to the par- 
 orchis. 
 
 The latter consists of greatly con- 
 voluted canals, and from it arises 
 the vas deferens (Wolffian duct), 
 which either takes a straight course, 
 or is more or less coiled. In Birds 
 it opens by an independent aperture 
 into the cloaca, while in Lizards it 
 fuses with the ureter shortly before 
 entering the latter. 
 
 Remains of the Miillerian ducts are 
 present in the male, corresponding in 
 position with those of the female. Their 
 lumen is not continuous throughout, but 
 the abdominal aperture may remain open 
 (Emys europsea), and exceptionally (e.g. 
 Lacerta viridis) they may be as well de- 
 veloped as in the female. 
 
 Lymphoid organs are present in many 
 Reptiles, and probably have a physiological 
 relation to the generative organs (compare 
 p. 368). In many Lizards they are large 
 and variously coloured, and lie within the 
 pelvic region ; in Snakes they extend 
 along almost the entire body-cavity. 
 
 Herniaphroditism has been observed 
 very exceptionally in the Chaffinch. Occa- 
 sionally the ovary of some Birds undergoes 
 structural changes, and no longer produces 
 ova, the female then taking 011 certain 
 secondary sexual characters of the male. 
 
 FIG. 295. MALE URINOGENITAL 
 ORGANS OF Anguis fragilis. 
 (After F. Leydig.) 
 
 Ho, testis; f, the so-called "yellow 
 body " (suprarenal) ; Ep, paror- 
 chis ; Vd, vas deferens ; p, p, 
 common aperture of the ureter 
 (Ur, Ur l ) and vas deferens on a 
 papilla on the dorsal wall of the 
 cloaca (C7) ; B, urinary bladder ; 
 r, rectum ; N, kidney ; mg, rudi- 
 ment of the Miillerian duct. 
 
 Mammals. In Mammals the generative apparatus no longer 
 extends along the entire body-cavity, as in the lower Vertebrates, 
 but is confined to the lumbar and pelvic regions. Moreover, 
 in correspondence with the close relations which usually take 
 place between mother and embryo (p. 338), there is a much greater 
 differentiation of the generative organs than occurs in lower types. 
 The transition is not, however, a sudden one, for in the lowest 
 Mammals, viz., the Montremes and Marsupials (Figs. 296 and 297), 
 
URINOGENITAL ORGANS 
 
 371 
 
 these organs show many points of resemblance with those of Reptiles 
 and Birds. 
 
 FIG. 296. A, MALE URINOGENITAL ORGANS OF Ornithorhynchus paradoxus ; B, 
 FEMALE URINOGENITAL ORGANS OF Echidna hystrix. 
 
 N, kidneys ; t/r, ureter ; B, urinary bladder ; Sug, urinogenital canal ; Ho, 
 testis ; Ve, vas efferens ; Ep, epididymis ; Vd, vas deferens ; Od, oviduct ; 
 r, rectum ; Cl, cloaca, opening to the exterior at Cl 1 ; m to m 4 , muscles of the 
 cloaca and penis (or clitoris) ; Gp, glans penis, enclosed within its fibrous 
 tube ; Pp, prepuce ; Cli, clitoris ; *, aperture through which the copulatory 
 organ extends into the cloaca. 
 
 Thus in the oviparous Monotremes the left ovary is more strongly 
 developed than the right, and each has the appearance of a bunch 
 of grapes ; the cloaca persists, and the oviducts (Miillerian ducts), 
 
 B B 2 
 
jr.. 
 
 TIG. 297. FEMALE GENERATIVE APPARATUS OF CERTAIN MARSUPIALS. A, 
 Didelphys dorsigera (juv.) ; B, Phalangista vidpina ; C, Phascolomys ivombat. 
 (After A. Brass'.) 
 
 .NN, kidneys ; Ur, ureters ; Ov, ovary ; Ot (Fim), abdominal opening of 
 Fallopian tube ; Od, oviduct ; Ut, uterus ; Ut l , openings of uteri into the 
 vaginal caecum, VgB ; f, bend between uterus and vagina, Vy ; Vg l , apertures 
 of vaginae into the urinogenital canal (Sug) ; B, urinary bladder ; r, rectum, 
 which opens to the exterior (Cl) at r l ; g, clitoris ; *, t, rectal glands. 
 
URINOGENITAL ORGANS 373 
 
 which in other Mammals become more or less fused with one 
 another proximally, remain distinct throughout, and open into the 
 urinogenital canal anteriorly to the ureters and bladder. 
 
 In the higher Mammals the oviducts become distinctly differ- 
 entiated into three portions, a Fallopian tube, a uterus, and a 
 vagina. The vagina opens to the exterior (Figs. 273, 297, 298), 
 while the Fallopian tube communicates with the abdominal cavity 
 by a ciliated funnel-shaped aperture. 
 
 In Marsupials the fusion of the two oviducts is much less 
 marked than in the higher Mammals, and in order to trace 
 the gradual differentiation of these parts, their condition in Opos- 
 sums (Didelphidae) must now be described in greater detail. 
 
 A dilated portion of each oviduct (Fig. 297, A), giving rise to a 
 uterus, is plainly distinguishable from the rest, and its narrowed 
 posterior end comes into close contact with its fellow in the middle 
 line. At this point (f) each uterus communicates with the vagina 
 by a distinct os uteri. The vagina curves sharply outwards, and, 
 then backwards, opening close to its fellow into the elongated urino- 
 genital canal. The ureters, as in all other Marsupials in which the 
 vaginae have a similar arrangement, pass between the curved 
 portions of the vaginas to the bladder. 
 
 From the condition of the female generative organs in Didel- 
 phys, that seen in other Marsupials can be easily explained. In 
 Phalangista vulpina and Phascolomys wombat (Fig. 297, B and c) 
 the anterior ends of the knee-shaped bends of the vaginae lie 
 closer together, and begin to extend backwards towards the urino- 
 genital canal, the septum between them disappearing at the 
 same time. A vaginal ccecum is thus formed : this may 
 become more elongated, arid finally extend backwards so as to 
 meet the anterior wall of the urinogenital canal, into which it 
 may open by the formation of a so-called third vagina. The 
 anus and urinogenital apertures are surrounded by a common 
 sphincter. 
 
 In the placenta! Mammals the posterior portions of the Miiller- 
 ian ducts become fused together to form an unpaired vagina, and a 
 definite cloaca exists only in the embryo, although even in the 
 adult in certain cases (e.g., amongst Rodents), the anus and urino- 
 genital aperture may be enclosed by a common fold of the integu- 
 ment as in Marsupials, and a median septum may be present in 
 the vagina distally, indicating its primary double nature. The 
 uterine portions of the oviducts may also fuse with one another to 
 a greater or less extent, and thus the most various forms of uteri 
 result (uterus duplex, bicornis, bipartitus, and simplex}, as shown in 
 Fig. 298, A to D. The Primates possess a single uterus, and in this 
 case the primitively paired condition of the Mtillerian ducts is seen 
 only in the Fallopian tubes. The latter vary much in form, and 
 their abdominal apertures are usually provided with fringe-like 
 appendages (fimbriae). The ureters, unlike those of Marsupials, 
 
374 
 
 COMPARATIVE ANATOMY 
 
 Oct Od 
 
 ILw 
 
 B 
 
 I) 
 
 FIG. 298. VARIOUS FORMS OF UTERI. A, B, C, D, diagrams showing the 
 different stages in the fusion of the Miillerian ducts : A, uterus duplex ; B, 
 uterus bicornis ; C, uterus bipartitus ; D, uterus simplex ; E, female urino- 
 genital apparatus of Mmtdiiia, containing embryos (*, *) in the uterus ; F, 
 ditto of Hedgehog (Erinaceus). 
 
 Od, Fallopian tube ; Ut, uterus ; Vg, vagina ; Ce, cervix uteri ; Of, abdominal 
 aperture of Fallopian tube ; t, f, accessory glands ; r, rectum ; Sny, urino- 
 genital canal ; K, kidney ; Nn, adrenal ; Ur, ureter ; B, urinary bladder. 
 
 always pass to the outer sides of the genital passage, the vagina 
 being single. 
 
 The urinogenital canal may, as in Marsupials, be of a consider- 
 able length (e.g. amongst Rodents), and a fold of the mucous 
 
URINOGENITAL ORGANS 375 
 
 membrane (hymen) 1 is often present where the vagina opens into 
 it. On the ventral (anterior) wall of the urinogenital canal, the 
 clitoris (Pp. 380-384) is situated. In both male and female the 
 space between the urinogenital aperture and the anus is known as 
 the perinccum. 
 
 The ovaries are usually small, and rounded or oval in 
 shape, their surface being either smooth, irregular, or furrowed. 
 The point at which the nerves and vessels enter is not covered 
 by peritoneum, and is called the hilum. 
 
 Remains of the mesoiiephros, known as the parovarium, are present in 
 the neighbourhood of the ovary, oviduct, and uterus. These usually consist 
 of small crecal tubes, forming a network, which are connected together by 
 a collecting duct. In cases where the Wolffian duct persists in the female, 
 it passes from the parovarium to the urinogenital sinus and is spoken of as 
 Gartner's duct (Fig. 273, H). 
 
 A fold of 'the skin of the abdomen forming a pouch or mar-, 
 supium is present in Echidna and to a greater or less extent 
 in Marsupials (p. 28) : this serves to protect the egg (Echidna) 
 and young, which in the latter are born in a very unripe con- 
 dition, thus rendering possible a longer connection between the 
 mother and embryo during lactation. The aperture of the marsu- 
 pial pouch opens anteriorly or posteriorly, according to the mode of 
 life of the animal, and is provided with a muscle capable of clos- 
 ing it. In Marsupial embryos the margins of the lips become 
 partially fused secondarily and temporarily to form a suctorial 
 mouth, by means of which the young, many of the organs of which 
 are still in a " larval " condition, become attached to the teats 
 (compare p. 288). 
 
 In male Mammals, the testes arise in the same relative position 
 as the ovaries of the female. The ovary, however, never moves in 
 the course of development further backwards than the pelvis; but 
 the testis may pass out of the abdomen through an inguinal 
 canal into a purse-like outgrowth of the integument in the inguinal 
 region called the scrotal sac, which is lined by a continuation of the 
 peritoneum, the tunica vaginalis. The two scrotal sacs (which are 
 represented amongst female Primates by the so-called labia majora 
 of the vulva) may remain separate, or unite to form a scrotum : in 
 Marsupials this is situated in front of, and in placental Mammals 
 behind the penis (p. 382). If the inguinal canals remain widely 
 open, they may be withdrawn periodically into the abdomen (as e.y 
 in Rodentia and Insectivora, in which they only descend at sexual 
 maturity) ; this is effected by means of the cremaster muscle, a con- 
 tinuation of the fibres of the internal oblique and transversalis, or of 
 the latter only, and corresponding t-o the " compressor mammne " of 
 Marsupials. When the inguinal canals become reduced, the testes 
 remain throughout life in the scrotum. In many Mammals, how- 
 
 1 A similar fold, closing the apertures of the oviducts in the immature condi- 
 tion, is present in Elasmobranchs. 
 
376 
 
 COMPARATIVE ANATOMY 
 
 ever (e.g., Monotremes, most Edentates, Elephant), the testes remain 
 permanently within the abdomen. 
 
 The testes are smooth, and somewhat oval in form, and their 
 relative size varies in different Mammals ; they are covered by a 
 
 FIG. 299. MALE UBINOOENITAL APPARATUS OF THE HEDGEHOG (Erinaceus).. 
 
 N, kidney ; Ur, ureters ; B, urinary bladder ; Pm, membranous portion of 
 urinogenital tube ; Cpc, corpus cavernosum ; Pp, prepuce ; Gp, glans penis ; 
 PD, preputial glands ; Cd, Cowper's glands ; Pr, Pr l , the different lobes of 
 the prostate ; Sb, vesicula seminalis ; Ho, testis ; Ep, epididymis ; Vd, Vd^y. 
 vas deferens. 
 
 fibrous investment (Fig. 300), from which processes (trabeculse) 
 usually extend inwards. Thus the seminal tubules become 
 separated into definite lobed masses, and a sort of lattice-work is 
 also formed (corpus Highmori), through which the elements of 
 the vasa efferentia pass to the epididymis. In the latter the 
 
COPULATORY ORGANS 
 
 377 
 
 seminal tubules become rounded off to form the so-called coni 
 vasculosi, and these are connected together by a collecting duct, 
 the vas epididymitis. The vas deferens arises from the last 
 conus vasculosus, and gives rise towards its distal end, shortly 
 before it opens into the urinogenital sinus close to an elevation the 
 colliculus seminalis, to glandular outgrowths (vesiculce seminales), 
 which may attain a relatively enormous size in Rodents and In- 
 sectivores (Fig. 299). From this point to its termination at the 
 
 FIG. 300. DIAGRAMMATIC SECTION OF THE TESTIS OF A MAMMAL. 
 
 Ho, testis ; NH, epididymis ; Vd, vas deferens ; A , albuginea of the testis, which 
 gives rise to the trabeculse (t, t) and the corpus Highmori (f) ; L, L, coils of 
 the seminal tubules ; Ve, vasa efferentia (rete Halleri) ; Cv, coni vasculosi, 
 which are connected together by the collecting duct, Vep ; Va, vas aberrans. 
 
 apex of the penis, the seminal canal is spoken of as the ductus 
 ejaculatwius. 
 
 In many Mammals rudiments of the Miillerian ducts are pre- 
 sent in the male, and open into the urinogenital sinus. In Man, 
 only the most posterior end of the ducts remain in the form of an 
 unpaired vesicle (uterus masculinus), which lies embedded within 
 an accessory genital gland, the prostate (Fig. 299). This gland, which 
 more or less completely surrounds the urinogenital sinus, consists 
 of glandular tubules connected together by means of fibrous and 
 muscular tissue : its secretion is poured into the urinogenital sinus, 
 and is apparently of great importance in connection with the 
 activity of the spermatozoa. 
 
 Copulatory Organs. 
 
 Various forms of copulatory organs, morphologically distinct 
 from one another, occur amongst Vertebrates. In male Elasmo- 
 branchs a specially modified portion of each pelvic fin (clasper or 
 pteryyopodium, p. 122) serves this purpose. It consists of a series 
 
378 
 
 COMPARATIVE ANATOMY 
 
 of more or less calcified cartilages, covered by skin and muscles, 
 which are movable upon one another and are derivatives of the 
 fin-rays, and it is provided with a channel along the inner side. 
 These claspers are inserted, in a closed condition, into the cloaca 
 of the female, and thence into the oviducts ; they are then opened 
 out by means of special muscles, and the seminal fluid flows along 
 their channels into the distended oviducts. In connection with 
 this apparatus there is a gland (p. 18) surrounded by muscular 
 fibres, which is formed as an involution of the integument; in 
 its histological character this calls to mind the uropygial gland of 
 Birds (p. 21). 
 
 In addition to pter}'gopodia, similar to those of Elasmobranchs 
 the Holocephali possess a pair of curious anterior claspers 
 (Fig. 301), which are protruded from a shallow pouch situated 
 
 Fio. 301. PELVIC ARCH AND SKELETON OF PELVIC FIN AND CLASPER OF A 
 MALE Chimcera monstrosa. (After Da vidoff. ) Ventral view. 
 
 Pr.il, B, dorsal part of pelvic arch (iliac process) ; S.B, anterior clasp er ; aj, 
 1 3, the various segments of the posterior clasper ; Mt, basipterygium ; 
 Ra, Ra l , radii of fin. 
 
 in front of the pelvic fins ; each of these consists of a plate 
 covered with dermal denticles, and in Callorhynchus a grooved 
 structure is present in addition. 1 There is also a knob-like organ, 
 usually known as the frontal clasper, on the upper surface of the 
 head. (Fig. 57). 
 
 1 These may correspond to portions of the primitive lateral fin-folds (p. 103) 
 which are not represented in other Craniata. 
 
COPULATORY ORGANS 
 
 379 
 
 Amongst the Teleostei, the terminal portion of the anal fin in 
 the male Girardinus is modified to form an apparatus for holding 
 the female, and in many Cyprinoids certain modifications occurring 
 in the anal fin may have a similar function. 
 
 Amongst the Amphibia, a marked swelling of the lips of the 
 cloaca may occur in Urodeles during the breeding season and inter- 
 nal impregnation may take place, but only in the male Gymno- 
 phiona is a definite copulatory organ present ; this simply consists 
 of the eversible cloaca, which is regulated by a well-developed 
 musculature (Fig. 302). 
 
 m.r.cl 
 
 FIG. 302. THE POSTERIOR PART or THE URINOGKNITAL APPARATUS OF A, 
 Epicrium ylutinosum AND B, CcKcilia lumbricoides. 
 
 Cl, Cl l , CI-, the different sections of the cloaca, and BJ, its ceecal processes : the 
 cloaca is shown retracted in A and everted in B ; els, cloacal sheath ; m.r.d, 
 retractor muscle of the cloaca ; B, B 1 , the two horns of the urinary bladder ; 
 N, kidneys ; Ig, Wolffian duct ; mg, Miillerian duct ; r, rectum ; Mdg, cloacal 
 aperture ; HS, scales in the integument. 
 
 Two kinds of copulatory organs are found in Reptiles, the one 
 being seen in Lizards and Snakes, and the other in Chelonians and 
 Crocodiles. 
 
 In the former there are two copulatory sacs or penes lying outside 
 the cloaca, under the skin at the root of the tail, and these can be 
 everted and protruded through the vent and again withdrawn by 
 means of a muscle inserted into the blind end of the sac (Fig. 303). 
 In its everted condition, a spiral furrow extends along each sac down 
 
380 
 
 COMPARATIVE ANATOMY 
 
 which the seminal fluid passes. These organs are also represented 
 in the female, in which they are, however, much smaller. 
 
 In Chelonians and Crocodiles the penis is single, and corre- 
 sponds to a thickened portion of the ventral wall of the cloaca 
 
 
 FIG. 303. COPULATORY ORGANS OF Lacerta agilis. ( After F. Leydig.) In A they 
 are shown everted, and in B their position in the retracted condition is indicated 
 by dotted lines extending backwards from the vent (Ce). 
 
 R, Penes ; f, spinal furrow ; SD, the so-called "femoral glands " (see p. 20). 
 
 (Figs. 304, 305, A). It consists of fibrous and cavernous (erectile) 
 tissue, and is protrusible, being regulated by muscles. In the 
 female it is represented by a smaller clitoris. The penis bifurcates 
 
 FIG. 304. TRANSVERSE SECTION OF THE CLOACA OF A CHELONIAN. (Slightly 
 diagrammatic. ) After Boas. 
 
 /, fibrous body ; r, seminal furrow, bounded by cavernous tissue ; v, wall of 
 
 cloaca. 
 
 proximally, and its distal tongue-shaped portion ends freely ; a 
 longitudinal groove extends along the upper surface, at the proxi- 
 mal end of which the vasa deferentia open. In Crocodiles the 
 free portion is relatively longer and the groove deeper than in 
 Chelonians. 
 
 In many Birds a copulatory organ is present, formed on a 
 similar plan to that of Chelonians and Crocodiles. It is well 
 developed amongst the Ratitae and La,mellirostres, and in many 
 other Birds can be recognised in a rudimentary condition. In 
 Struthio it resembles that of the Crocodile, and is supported by a 
 
A 
 
 B 
 
 FIG. 305. DIAGRAMMATIC LONGITUDINAL SECTIONS OF THE POSTERIOR PART OF 
 THE RECTUM, THE CLOACA, AND THE COPULATORY ORGANS OF VARIOUS 
 VERTEBRATES. (After Boas.) The position of the ureter and vas deferens 
 are indicated, although not situated in the median line. 
 
 A, Crocodile ; B, hypothetical form between A and C ; C, Monotreme (penis 
 
 extruded) ; D, Monotreme (penis retracted). 
 bi, connective- tissue ; bl, urinary bladder ; cl, cloaca ; d, rectum ; /, fibrous body 
 
 (corpora cavernosa of human anatomy) ; ps, sheath of penis ; ps l , aperture of 
 
 the sheath ; r, seminal furrow or tube (penial urethra) ; s, vas deferens ; u, 
 
 urinogenital canal. 
 
382 COMPARATIVE ANATOMY 
 
 fibrous body, bifurcated at the base. In Dromaeus and Rhea there 
 is an aperture at the apex of the penis leading into an elongated 
 and curved blind sac, in which is a furrow, lined by cavernous 
 tissue, continuous with the groove on the dorsal side of the 
 organ. In the D.uck and Swan the spiral penis is essentially 
 similar to that of Dromseus and Rhea. The absence of the blind 
 sac in the Ostrich, however, is probably a secondary modification. 
 A clitoris is present in the female of the above-mentioned Birds. 
 
 The penis of Monotremes may be best understood by imagin- 
 ing a hypothetical form intermediate between it and that of Croco- 
 diles and Chelonians (Fig. 305, B). We must suppose that 
 a sac-like outgrowth into which the ureter and vasa defe- 
 rentia open has become developed from the ventral cloacal wall at 
 the base (anterior end) of the penis, the groove in which has 
 become converted into a canal. The Monotreme condition is 
 reached by the sac elongating to form a urinogenital canal, into 
 the distal end of which the urinary and genital ducts and the 
 bladder open (c, D). The penis consists of an unpaired . fibrous 
 body enclosing the canal, and is only loosely surrounded by the 
 mucous membrane of the cloaca, so that it can be protruded from 
 and retracted into a sheath in which the apex or glans lies. 
 
 In Echidna, cavernous tissue is present in the glans ; and in Ornitho- 
 rhynchus the latter is bifurcated and covered with soft spines, the seminal 
 canal opening in a groove on each fork by numerous fine canals situated on 
 papillae. 
 
 A clitoris is present in the female of all Mammals. 
 
 In Marsupials (Fig. 306, A), the penis-sheath opens directly 
 on to the surface of the body ; the opening of the urinogenital 
 canal into the cloaca has become closed, and is continuous with 
 the seminal tube or urethra of the penis. The fibrous body 
 is paired, and both it and the walls of the penial urethra are com- 
 posed of cavernous tissue. 
 
 Amongst Flacental Mammals the penis of Rodents (Fig. 
 306, B) and Insectivores comes nearest to that of Marsupials. The 
 paired fibrous body (corpus cavernosum) bifurcates proximal ly 
 to form two crura, which are nearly always attached to the 
 ischia. The opening of the penis-sheath gradually becomes further 
 separated from the anus, and is situated more on the ventral side of 
 the body (compare B and c), the penis itself lying horizontally 
 along the abdomen. In Primates the organ becomes more or less 
 free from the body-wall, and either its distal end (Apes, c), or the 
 whole of it (Man, D) hangs freely, and the sheath forms a double 
 tube-like investment, the foreskin or prepuce, over the glans. 
 
 In the course of development, the penis of Marsupials and 
 Placental Mammals passes through stages which resemble succes- 
 sively those which are permanent in Crocodiles and Chelonians 
 and in Monotremes. It arises from a " genital prominence" on the 
 
FIG. 306. Continuation of Fig. 305. For description see next page. 
 
384 
 
 COMPARATIVE ANATOMY 
 
 FIG. 306. (The special thickening of the corpus spongiosum and the glans penis 
 present in some Mammals is not indicated). 
 
 A, Marsupial (very diagrammatic, for comparison with Fig. 305 C ; the obliterated 
 opening of the urinogenital canal into the cloaca is indicated by dotted lines); 
 B, Rodent (Ccelogenys paca) ; C, Ape (Cercopitheai*) : in most placental Mam- 
 mals the apex of the penis does not hang down ; D, Man ; E, Human foetus. 
 
 Additional letterings : a, anus ; 6, pelvic symphysis ; p, genital prominence, which 
 gives rise to the penis or clitoris ; al, stalk of allantois. 
 
 ventral wall of the cloaca. A channel passes along the side facing 
 the cloaca to the opening of the urinogenital sinus : this condition 
 is usually retained throughout life in the case of the clitoris of the 
 female, while in the male (and occasionally in the female also) the 
 groove becomes closed to form a canal continuous with the 
 urinogenital canal or urethra, which thus becomes considerably 
 lengthened. In addition to the paired erectile corpora cavernosa 
 there is a median corpus spongiosum or corpus cavernosum urethrce 
 in connection with the penis (Fig. 307): corpora cavernosa are also 
 
 A 
 
 Ge- 
 
 FIG. 307. A, SEMIDIAGRAMMATIC FIGURE OF THE HUMAN PENIS. (In transverse 
 section and from the side.) B, CLITORIS OF A MONKEY (Cebus capucinus). 
 
 A, albuginea penis ; A 1 , albuginea urethrse ; Sp, septum between the two corpora 
 cavernosa ; S, sulcus dorsalis penis ; Ccp, corpus cavernosum ; Ccu, corpus 
 spongiosum, which gives rise to the glans penis at Gp, and forms an oval 
 enlargement (bulbus) at B ; rd, rd 1 , crura of the corpora cavernosa ; Cli, 
 clitoris, with its ventral furrow (R), glans (Gc), and prepuce (Pp). 
 
 present in the clitoris, and the corpus spongiosum, which retains 
 its paired character, is represented by the so-called bulbi vestibuli 
 at the vulva or entrance to the vagina. 
 
 In many Mammals a bone (os penis) becomes developed in 
 the septum between the corpora cavernosa (e.g., many Marsupials, 
 Rodents, Bats, Carnivores, Whales, Lemurs, and Apes). In some 
 {e.g., Seal) there is an os clitoridis in the female also. The glans is 
 provided with a special kind of tactile corpuscles, and in the male 
 may bear horny papillae and even calcified plates and spines (e.g., 
 certain Rodents). 
 
SUPRARENAL BODIES 385 
 
 In addition to the glandular vesiculse seminales and the 
 prostate (Fig. 377), paired Cmcper's glands (Fig. 299), open in the 
 male into the urinogenital canal, and representatives of these 
 (glands of Bartliolini) usually occur in the female. Preputial 
 glands are also present between the prepuce and glans penis and 
 in a corresponding position in the female. 
 
 SUPEARENAL BODIES. 
 
 The suprarenals or adrenals are bodies of a glandular structure 
 situated in the ccelome right and left of the vertebral column, 
 generally in close proximity to the kidneys. 
 
 Nothing is known of these bodies in Amphioxus or in the 
 Dipnoi. In Cyclostomes they are said to be present and to arise in 
 connection with the anterior part of the pronephros. In 
 Elasmobranchii and Holocephali they are represented by two- 
 distinct sets of structures paired or unpaired interrenals of a 
 yellow colour, close to the kidneys (Fig. 290), and a segmentally 
 arranged row of suprarenal bodies, situated close to the inter- 
 costal arteries in the neighbourhood of the kidneys. The former 
 apparently represent the cortical (mesoblastic) and the former the 
 medullary (epiblastic) portions of the adrenals of higher forms. 
 In Teleostei the adrenals are usually paired and are in close 
 relation with the kidneys they correspond to the interrenals 
 of Elasmobranchs. Amongst Ganoids the Sturgeon possesses 
 numerous yellow bodies of the same nature. In Amphibia these 
 organs form yellow streaks or dots on the ventral surface or inner 
 border of the kidneys, receiving blood from the renal portal veins 
 as well as from the renal arteries. 
 
 In the higher Vertebrates the adrenals consist of " cortical " 
 and " medullary " portions, the latter derived from the sympathetic 
 nervous system, and therefore epiblastic, and the former from the 
 pronephros or mesonephros, or from the germinal epithelium 
 (mesoblastic) : the mode of development of the cortical substance, 
 however, requires further investigation. They are abundantly 
 supplied with blood-vessels and must have an important function, 
 but their physiology is not understood. In Reptiles and Birds they 
 are elongated and lobulated, and are situated close to the gonads. 
 Both medulla and cortex are apparently represented in all these 
 forms, but the relative relations of the two parts vary greatly. 
 In Mammals each adrenal forms a definite and uniform rounded 
 or oval mass lying near the corresponding kidney (Fig. 285 B), the 
 medullary substance being central, and the cortical substance 
 peripheral. In many Mammals these organs contain pigment cells- 
 as well as numerous lymphatic follicles and vessels. 
 
 c c 
 
APPENDIX 
 
APPENDIX. 
 
 BIBLIOGRAPHY. 
 
 List of the more important periodicals dealing with Comparative Anatomy, 
 Embryology, Histology, &c. 
 
 (The abbreviations used in the following pages are given in brackets.) 
 
 Abhandlungen und Monatsberichte der K. Preuss. Akademie der Wissenschaften 
 
 zu Berlin. (Abh. Ale. Berlin.) 
 Abhaudlungen der mathematisch-physikalischen Classe der K. Bayerischen 
 
 Akademie der Wissenschaften. Miinchen. (Abh. Bayer. Ale.) 
 Abdhandlungen der mathematisch-naturwissenschaftlichen Classe der K. Bohm- 
 
 ischen Gesellschaft. Prag. (Abh. Bohm. Ges.) 
 Abhandlungen der K. Gesellschaft der Wissenschaften zu Gottingen. (Abh. Ges. 
 
 Gottingen. ) 
 
 Abhandlungen der naturforschenden Gesellschaft zu Halle. (Abh. Ges. Halle.) 
 Abhandlungen herausgegeben von der Senckenbergischen naturforschenden 
 
 Gesellschaft. Frankfurt-a-M. (Abh. Senckenb. Ges.) 
 American Journal of Science. New Haven. (Amer. J. Sci.) 
 ,- American Naturalist. Philadelphia. (Amer. Nat.) 
 
 Anatomischer Anzeiger. Centralblatt fiir die gesammte wissenschaftliche 
 
 Anatomic. Amtliches Organ der anatomischen Gesellschaft. Jena. 
 
 (Anat. Anz.) 
 Anatomische Hefte. Wiesbaden. Erste Abtheilung Arbeiten aus anatomischen 
 
 Instituten (Anat. Hefte Arb.): Zweite Abtheilung Ergebnisse der Anatomic 
 
 und Entwickelungsgeschichte. (Anat. Hefte Ergebn.) 
 . Annals and Magazine of Natural History. London. (Ann. Nat. Hist.) 
 
 * Annales des Sciences naturelles. Zoologie et Palaeoiitologie. Paris. (Ann. Sci. 
 
 Nat.) 
 
 Arbeiten aus dem zoologischen Institut der Universitat Wien. (Arb. Inst. Wien.) 
 Arbeiten aus dem zoologisch-zootomisch. Institut in Wiirzburg. Wiesbaden. 
 
 (Arb. Inst. Wiirzburg.) 
 Archiv fiir Anatomic und Physiologic. (Arch. f. Anat. u. Physiol.). l te , 
 
 Abtheilung Archiv fiir Anatomic und Entwickelungsgeschichte. (Arch. 
 
 Anat.) ; 2 te , Abtheilung- Archiv fiir Physiologic. Leipzig. (Arch. Physiol.) 
 
 * Archives de Biologic. Gand & Paris. (Arch. Biol.) 
 
 Archiv fiir Entwickelungsmechanik der Organismen. Leipzig. (Arch. Entwick. 
 
 Mechanik.) 
 
 o Archiv italienne de Biologic. Turin. (Arch. ital. Biol.) 
 Archiv fur mikroscopische Anatomic und Entwickelungsgeschichte. Bonn. 
 
 (Arch. f. mikr. Anat.) 
 
 t Archiv du Museum d'Histoire naturelle. Paris. (Arch. Mus. Paris.) 
 Archiv fiir Naturgeschichte. Berlin. (Arch. f. Naturgesch.) 
 
390 APPENDIX 
 
 I Archives de Zoologie experimentale et generale. Paris. (Arch. Zool. exp.) 
 Atti della R. Accademia dei Lincei. Memorie e Rendiconti. (Atti Ace. Lincei.)\ 
 Q Atti della Societa Toscana di Scienze naturali. Pisa. (Atti Soc. Toscana.) 
 a Atti della R. Accademia delle Scienze di Torino. (Atti Ace. Torino.) 
 
 Atti della Societa Ligustrica di Scienze naturali e geografiche. Geneva. (Atti' 
 
 Soc. Ligust..) 
 
 Berichte der naturforschenden Gesellschaft zu Freiburg i.B. (Ber. Ges. Freiburg.) 
 Biologisches Centralblatt. Leipzig. (Biol. Centralbl.) 
 Biologische Untersuchungen von Prof. Dr. Gustav Retzius. Stockholm. ( Biof. 
 
 Untersuch. ) 
 (7 Bolletino dei Musei di Zoologia e Anatomia comparata della R. University di 
 
 Geneva. (Boll. Mus. Genova.) 
 \ Bulletin de 1' Academic imperiale des Sciences de St. Petersbourg. (Bull. Ac. St. 
 
 Petersb.) 
 ' Bulletin de 1' Academic royale des Sciences de Belgique. Bruxelles. (Bull. Ac. 
 
 Belgique. ) 
 
 ^Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, 
 Mass. (Bull. Mus. Harvard.) 
 
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 /Bulletin de la Socie"te beige de Geologic et Palaeontologie. Bruxelles. (Bull. 
 
 Soc. beige Geol. ) 
 
 I Bulletin de la Socie"te zoologique de France. Paris. (Bull. Soc. Zool. France. ) 
 1 Bulletin de la Societe imperiale des Naturalistes de Moscou. (Bull. Soc. Moscou.) 
 I Comptes rendus hebdomadaires des Stances de 1' Academic des Sciences. Paris. 
 
 (Comptes rendus.) 
 Denkschriften der Kaiserl. Akademie der Wissenschaften zu Wien. (DenL AL 
 
 Wien.) 
 
 Internationale Monatschrift fur Anatomic und Histologie (Monthly Journal of 
 Anatomy and Physiology). Leipzig, London, Paris. (Int. Monatschr. 
 Anat.) 
 Jahresberichte iiber die Fortschritte der Anatomic und Physiologic. Bonn. 
 
 (Jahresber. Anat. u. Physiol.) 
 
 Jenaische Zeitschrift fiir Naturwissenschaft. Jena. (Jen. Zeitschr.) 
 I Journal de 1'Anatomie et de la Physiologic normales et pathologiques de 1'Homme 
 
 et des Animaux. Paris. (Journ. de VAnat. et Physiol.) 
 *- Journal of Anatomy and Physiology, normal and pathological. London. (Journ. 
 
 Anat. and Physiol.) 
 Journal of the College of Science, Imperial University, Japan. Tokyo. (Journ. 
 
 Coll. Sci. Japan.) 
 
 *- Journal of the Linnean Society : Zoology, London. (Journ. Linn. Soc. ) 
 - Journal of Morphology. Boston. (Journ. Morphol. ) 
 \ Me"moires de 1' Academic des Sciences de 1'Institute de France. Paris. (Mem. 
 
 Ac. France.) 
 VM&noires de PAcademie imperiale des Sciences de St. Petersbourg. (Mem. Ac. 
 
 St. Pdtersb.) 
 * Memoirs of the Museum of Comparative Zoology at Harvard College, Cambridge, 
 
 U. S. (Mem. Mus. Harvard. ) 
 Mittheilungen aus der zoologischen Station zu Neapel. Berlin. (Mittheif. Zool. 
 
 Stat. Neapel.) 
 
 Morphologische Arbeiten. Jena. (Morph. Arb.) 
 Morphologisches Jahrbuch. Leipzig. (Morph. Jahrb. ) 
 
 Mathematische und naturwissentschaftliche Mittheilungen aus den Sitzungs- 
 berichten der Konigl. preuss. Akademie der Wissenschaften zu Berlin. 
 (Mat. AJc. Berlin, see also Sitz-ber. Ak. Berlin.) 
 Nachrichten von der Konigl. Gesellschaft der Wissenschaften zu Gottingen 
 
 (Nachr. Ges. Gottingen.) 
 
 Niederlandische Archiv fiir Zoologie. (Niederl. Arch.) 
 
 Nova Acta Academise Csesarere Leopoldino-Carolinje ( Verhandl. d. K. Leop. -Carol. 
 
 deutsch. Akad. d. Naturforscher). Halle. (N.ActaAcad. Cces. Leop. Car.) 
 
 Natural Science ; a Monthly Review of Scientific Progress. London, New York. 
 
 (Nat. Sci.) 
 <- Nature : a weekly illustrated Journal of Science. London. (Nature. ) 
 
 Paleeontographica. Beitrage zu Naturgeschichte der Vorzeit. Stuttgart. 
 (PalfKontoqr. ) 
 
APPENDIX 391 
 
 Philosophical Transactions of the Royal Society of London. (Phil. Trans. ) 
 Proceedings of the Academy of Natural Sciences of Philadelphia. (P. Ac. Philad.} 
 Proceedings of the American Academy of Arts and Sciences. Boston. (P. Amer. 
 
 Ac.) 
 
 Proceedings of the Linnean Society of London. (P. Linn. Soc.) 
 ~ Proceedings of the Linnean Society of New South Wales. Sydney. (P. Linn. 
 
 Soc. N.S.W.) 
 
 ^Proceedings of the Royal Irish Academy. Dublin. (P. Irish Ac.) 
 Proceedings of the Royal Society of London. (P. Roy. Soc.) 
 Proceedings of the Royal Dublin Society. (P. Dublin Soc.) 
 Proceedings of the Zoological Society of London. (P. Zool. Soc. ) 
 Quarterly Journal of Microscopical Science. London. (Q. Journ. Micr. Sci.) 
 ( Revue biologique du Nord de la France. Lille. (Rev. biol. Nord France.) 
 Sitzungsberichte du Konigl-Preuss. Akademie der Wissenschaften zu Berlin (Sitz. 
 
 ber. AL Berlin, and Mt. AL Berlin.) 
 Sitzungsberichte der Gesellschaft fiir Morphologic und Physiologic in Miinchen. 
 
 (Sitz. ber. Morph. Physiol. Miinchen.) 
 Sitzungsberichte der mathematisch-naturwissenschaftlichen Classe der K. 
 
 Akademie der Wissenschaften. Wien. (Sitz. ber. Ale. Wien.) 
 Sitzungsberichte der K. Bohmischen Gesellschaft der Wissenschaften. Prag. 
 
 (Sitz. ber. Bohm-Ges.) 
 Sitzungsberichte der Gesellschaft naturforschenden Freunde zu Berlin. (Sitz. ber. 
 
 Naturf. Berlin.) 
 Sitzungsberichte der physikalisch-medicinischen Gesellschaft zu Wurzburg. 
 
 (Sitz. ber. Ges. Wiirzburg.) 
 Science Progress : a Monthly Review of Current Scientific Investigation. London. 
 
 (Sci. Progress.) 
 Studies from the Morphological Laboratory of the University of Cambridge. 
 
 (Stud. Morph. Lab. Camb.) 
 Studies in Biology from the Biological Department of the Owens College, 
 
 Manchester. (Stud. Owens Coll. ) 
 
 Transactions of the Royal Dublin Society. (Tr. Dublin Soc.) 
 Transactions of the Royal Irish Academy, Dublin. (Tr. Irish Ac.) 
 Transactions of the Linnean Society of London : Zoology. (Tr. Linn. Soc.) 
 Transactions and Proceedings of the New Zealand Institute. Wellington. ( Tr. 
 
 N.Z. Inst.) 
 
 Transactions of the Royal Society of Edinburgh. (Tr. R. Soc. Edin.) 
 Transactions of the Zoological Society of London. (Tr. Zool. Soc.) 
 Verhandlungen. der Anatomischen Gesellschaft. (Verh. Anat. Ges., see Anat. 
 
 Anz.) 
 
 Verhandlungen der naturforschenden Gesellschaft in Basel. (Verh. Ges. Basel.) 
 Verhandlungen der k.k. zoologisch-botanischen Gesellschaft in Wien. (Verh. 
 
 Ges. Wien.) 
 Verhandlungen der k. Akademie der Wissenschaften zu Amsterdam (Verh. Ak. 
 
 Amsterdam.) (a tf-U 
 
 Verhandlungen der physikalisch-medicinischen Gesellschaft zu Wiirzburg (Verh. ^ 
 
 Ges. Wiirzburg.) - 5] 
 
 Zeitschrift fiir wissenschaftliche Zoologie. Leipzig. (Zeitschr. f. wiss. Zool. ) f~ * T/L 
 
 Zoologischer Anzeiger, zugleich Organ der deutschen zoologischen Gesellschaft. T 'f 
 
 Leipzig. (Zool. Anz.) ~ fe 
 
 Zoologische Jahrbticher, Abtheilung fiir Anatomic und Ontogenie der Thiere. f^ 
 
 Jena. (Zool. Johrb.) ^ 
 
 Zoologischer Jahresbericht, herausgegeben von der zoologischen Station zuNeapel. *^ 
 
 Berlin. (Zool. Jahresber.) P 
 
 Zoological Record. London. 
 
 GENERAL EMBRYOLOGY. 
 
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392 APPENDIX 
 
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APPENDIX 393 
 
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394 APPENDIX 
 
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 SIEBOLD, v., and STANNIUS. Handbuch der Zootomie. Berlin, 1854. (Handbuch 
 der Anatomie der Wirbelthiere Bd. I. Heft 1 2, containing the 
 Anatomy of Fishes, Amphibians and Reptiles.) 
 
 STRICKER, S. Handbuch der Lehre von den Geweben, etc. Leipzig, 1871. 
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 **t Braunschweig, 1883-94. (Also a French edition. ) 
 
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 1893. 
 
 ZITTEL, K. v. Handbuch der Palaontologie. Miincheii u. Leipzig. (Eng. trans. 
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 PAPERS, MONOGRAPHS, &c., DEALING WITH INDIVIDUAL TYPES 
 AND GROUPS OF ANIMALS. 
 
 AMPHIOXUS. 
 
 HATSCHEK, B. Studien iiber Entwicklung des Amphioxus. Arb. Inst. Wien, 
 
 1882. 
 LANGERHANS, P. Zur Anatomie des Amphioxus lanceolatus. Arch. f. mikr. 
 
 Anat. Bd. XII. 
 LANKESTER, E. RAY. Contributions to the knowledge of Amphioxus lanceolatus, 
 
 Q. Journ. Micr. Sci. (new Series). No. 124. Vol. 31. 1890. 
 LANKESTER and WILLEY. The development of the atrial chamber of Amphioxus : 
 
 Q. Journ. Micr. Sci. (new Series). No. 124. Vol. 31. 1890. 
 MULLER, J. Ueber . den Bau und die Lebenserscheinungen des Branchiostoma 
 
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 ROLPH, W. Untersuchungen iiber den Bau des Amphioxus lanceolatus. Morph. 
 
 Jahrb. Bd. II. 187(1 
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APPENDIX 395 
 
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 1891. 
 
 Amphioxus and the Ancestry of the Vertebrates. New York and London, 
 1894. 
 
 PISCES AND DIPNOI. 
 
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 Vol. XIV., 1878, and Vol. XX., 1884. 
 
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 Pelagic Stages of Young Fishes. Mem. Mus. Harvard. Vol. XIV. No. I, 
 part 1. 1885. 
 
 / AGASSIZ, L. Recherches sur les poissons fossiles. V. vol. avec atlas, 1833-43. 
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 BALFOUR, F. M. (See p. 391.) 
 
 BALFOUR, F. M. and W. N. PARKER. On the Structure and Development of 
 
 Lepidosteus. Phil. Trans. 1882. 
 
 BISCHOFF, TH. Lepidosiren paradoxa. Leipzig, 1840. 
 
 I BUJOR, P. Contribution a 1'etude de la Metamorphose cle I'Ammocoetes branchi- 
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 tfEMERY, C. Fierasfer. Studi intorno alia Sistematica, 1'Anatomia e la Biologia 
 
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 FRITSCH, A. Fauna der Gaskohle und der Kalksteine der Performation Boh- 
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 GARMAN, S. Chlamydoselachus anguineus, Garm. a living species of cladodont 
 
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 ^GUNTHER, A. Description of Ceratodus. Phil. Trans., 1871. 
 
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 L, F. R 
 
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 HYRTL, J. Lepidosiren paradoxa. Abh. bohm. Ges., 1845. 
 
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 KUPFFER, C. Die Entwicklung des Herings im Ei. Jahresbericht der Com- 
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 LANGERHANS, P. Untersuchungen itber Petromyzon planeri. Verh. Naturforsch. 
 
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 LEYDIG, FR. Beitriige zur mikrosk. Anatomic und Eiitw.-Geschichteder Rochen 
 
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396 APPENDIX 
 
 LEYDIG, FR. Zur Anatomic und Histologie der Chimaera monstrosa. Arch. f. 
 
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 MULLER, J. Ueber den Bau und die Grenzen der Ganoiden. Berlin, 1846. 
 
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 SCHNEIDER, A. Beitrage zur vergl. Anatomic und Entw.-Geschichte der Wir- 
 
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 SEMON, R. Zoolog. Forschungsreisen in Australien und dem malayischen 
 
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 SEMPER, C. Die Stammesverwandtschaft der Wirbelthiere und Wirbellosen. 
 
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 SHIPLEY, A. E. On some points in the Development of Petromyzon fluviatilis. 
 
 Q. Journ. Micr. Sci. Vol. XXVII. 1887. 
 
 TRAQUAIR, R. H. Papers on Palseospondylus Gunni, see Ann. Nat. Hist. (6), 
 y Vol. VI., 1890, and Proc. Roy. Phys. Soc. Edin. Vol. XII., 1893. 
 
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 ~/^? WIEDERSHEIM, R. Zur Biologic von Protopterus. Anat. Anz. , 1887. 
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 AMPHIBIA. 
 
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APPENDIX 397 
 
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 FISCHER, J. G. Anatomische Abhandlungen tiber die Perennibranchiaten und 
 
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 FRITSCH, A. (See p. 395.) 
 
 GAUDRY, A. LActinodon Arch. Mus. Paris, 1887. 
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 KLINCKOWSTROM, A. v. Zur Anatomic der Pipa americana. Zool. Jahrb. VII. 
 
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 LEYDIG, F. Die Anuren Batrachier der deutschen Fauna. Bonn, 1877. 
 
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 ^.^ MARSHALL, A. MILNES. The Frog. An introduction to Anatomy, Histology,. 
 
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 ^ RUSCONI, M. Histoire naturelle, developpement et metamorphose de la Sala- ^ 
 
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 SABASIN, P. and F. Ergebnisse naturwissenschaftl. Forschungeii auf Ce\ 7 lon in 
 
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 Zur Entwicklungsgeschichte und Anatomie der ceylonesischen Blind wiihle, 
 
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 WIEDERSHEIM, R. Bemerkungen zur Anatomie des Euproctus Rusconii. Annal. 
 
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 Labyrintliodon Riitimeyeri. Abh. Schweiz. Pal. Ges. Vol. V. 1878. 
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 Salamandrina perspicillata und Geotriton fuscus. Versuch einer vergl. 
 
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 REPTILIA. 
 
 BEMMELEN, J. F. VAX. Beitrage zur Kenntnis der Halsgegend bei Reptilien. 
 
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 Abh. Senckenb. Ges. Bd. XIII. 1883. 
 MARSH, O. C. The Dinosaurs of North America. Sixteenth Annual Report of 
 
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 MAURER, F. Die ventrale Rumpfmuskulatur einigen Reptilien. Festschrift fur 
 
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 MITSUKURI, K. On the Process of Gastrulation in Chelonia. Journ. Coll. 
 
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398 APPENDIX 
 
 ORLANDI, S. Note anatom. sal Macroscincus Coctei (Barb du Boc). Mus. di 
 
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 Das Skelet der Lacerta Simonyi, Steind. und der Lacertidenfamilie iiber- 
 
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 SMALIAN, C. Beitrage zur Anatomic der Amphisbaniden. Zeitschr. f. wiss 
 
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APPENDIX 399 
 
 MAMMALIA. 
 
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 FLOWER, W. H. Introduction to the Osteology of the Mammalia. 3rd ed. 
 
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 FLOWER and LYDEKKER. Mammals, living and extinct. London, 1891. 
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 GIACOMINI, C. Annotazioni sulla Anatomia del Negro. Fiinf Abtheilungen. 
 
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 GIEBEL, C. G. Die Saugethiere in zoologischer, anatomischer, und palaontolo- 
 
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 GULDBERG, G., and NANSEN, F. On the development and structure of the 
 
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 KOWALEWSKY, W. Sur FAnchitherium Aurelianense, Cuv. Mem. Ac. St.- 
 
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 LEISERING and MULLER. Handbuch der vergl. Anatomic der Haussaugethiere. 
 
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400 APPENDIX 
 
 LEYH. Handbuch der Anatomic der Hausthiere. 1850. 
 
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 QUAIN'S Elements of Anatomy. Edited by E. A. Schafer and G. D. Thane. (3 
 
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 SEWERTZOFF, A. Beitrag z. Ent. Gesch. d. Wirbelthierschadels. Anat. Anz. Bd. 
 
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 WIEDERSHEIM, R. Der Bau des Menschen als Zeugnis fiir seine Vergangenheit. 
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APPENDIX 401 
 
 WOEKS DEALING WITH INDIVIDUAL SYSTEMS OF 
 ORGANS. 
 
 A. INTEGUMENT. 1 
 
 (a) PISCES AND DIPNOI. 
 
 BOTTARD, A. Les Poissons venimeux, Contribution a 1' Hygiene navale. Paris, 
 
 1889. 
 FRITSCH, G. Die aussere Haut und die Seitenorgane des Zitterwelses (Malop- 
 
 terurus electricus). Sitz-Ber. Ak. Berlin. XXII. 1886. 
 Die elektrischen Fische. Leipzig, 1887 and 1890. 
 LANGERHANS, P. I" nters. liber den Bau des Amphioxus lanceolatus. Morph. 
 
 Jahrb. Bd. II., 1876. 
 
 LANKESTER, E. R. On the Lepidosiren of Paraguay, and on certain external 
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 LENDENFELD, R. v. Die Leuchtorgane der Fische. Biol. Centralbl. Bd. VII., 
 
 1887. 
 LEYDIG, F. Die augeniihnlichen Organe der Fische. Bonn, 1881. 
 
 Neue Beitr. zur anatomischen Kenntnis der Hautdecke und Sinnesorgane 
 
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 Integument brtinstiger Fische und Amphibien. Biol. Centralbl. Bd. XII. 
 
 1892. 
 LIST, J. Ueber Wanderzellen im Epithel. Zool. Anz. No. 198. VIII. Jahrg., 
 
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 PARKER, W. N. On the poison-organs of Trachinus. P. Zool. Soc., 1868. Anat. 
 
 Anz. III. Jahrg. 1888. 
 
 SACCHI, MARIA. Sulla struttura del tegumento negli embrioni ed avannotti del 
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 SCHULZE, F. E. Epithel- und Driisenzellen. Arch. f. mikr. Anat. Bd. III. 
 
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 (b) AMPHIBIA. 
 
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 EBERTH. Untersuchungen z. norm. u. path. Anat. d. Froschhaut. Leipzig, 
 
 1869. 
 FICALBI, E. Ricerche sulla struttura minuta della Pelle degli Anfibi (Hy- 
 
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 HALLER, B. Ueber das blaue Hochzeitskleid des Grasfrosches. Zool. Anz. No. 
 
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 1 Compare also under sensory organs. 
 
 D D 
 
402 APPENDIX 
 
 HEIDENHAIN, M. Ueber das Vorkommen von Intercellularbriicken zwischeii 
 glatteii Muskelzellen und Epithelzellen des aussereii Keimblattes und deren 
 theoretische Bedeutung. Anat. Anz. VIII. Jahrg., 1893. 
 Die Hautdriisen der Amphibien. Sitz. Ber. Ges., Wiirzburg, 1893. 
 HUBER, O. Ueber Brunstwarzen bei Rana temporaria. Zeitschr. f. wiss. Zool. 
 
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 JUNIUS, P. tiber die Hautdriisen des Frosches. Arch. Anat. Bd. XLVII. 
 
 1896. 
 LANGERHANS, P. Ueber die Haut der Larve von Salamandra maculosa. Arch. 
 
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 LEYDIG, F. Die Hautdecke und Hautsinnesorgane der Urodeleii. Morph. 
 
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 Bd. XII. 1876. 
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 MAURER, F. Glatte Muskelzellen in der Cutis der Anuren und ihre Beziehung 
 
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 NICOGLU, PH. Ueber die Hautdriisen der Amphibien. Zeitschr. f. wiss. Ziol. 
 
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 PAULICKI. Ueber die Haut des Axolotls. Arch, fiir mikr. Anat. Bd. XXIV. 
 
 1884. 
 
 PFITZNER, W. Die Epidermis der Amphibien. Morph. Jahrb. Bd. VI. 1880. 
 Die Leydig'schen Schleimzellen in der Epidermis der Larve von Salamandra 
 
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 SCHUBERG, A. Ueber den Bau und die Function der Haftapparate des Laub- 
 
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 Beitrag zur Kenntnis der Amphibienhaut. Zool. Jahrb. Bd. VI. 
 SCHULZ, P. Ueber die Giftdriisen der Kroten und Salamander. Arch. f. mikr. 
 
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 WIEDERSHEIM, R. Die Kopfdriisen der geschwanzten Amphibien und die 
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 ZALESKY. Ueber das Samandrin. Medic, chem. Untersuchungen, herausgegeben 
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 (c) REPTILIA. 
 
 BATELLI, A. Beitrage zur Kenntnis des Baues der Reptilienhaut. Arch. f. 
 
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 BLANCHARD, H. Recherches sur la structure de la peau des lezards. Bull. Soc. 
 
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 BRAUN, M. Zur Bedeutung der Cuticularborsten auf den Haftlappen der 
 
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 CARTIER, 0. Studien iiber den feineren Bau der Haut bei den Reptilien. 
 
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 FICALBI, E. Ricerche istologiche sul Tegumento dei Serpenti. Atti. d. Soc. 
 
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 Osserv. sulla Istologia della Pelle dei Rettili Cheloniani. Atti. d. R. 
 
 Accadem. dei Fisiocritici. Ser. IV. Vol. I. Siena, 1889. 
 GOPPERT, E. Zur Phylogenese der Wirbelthierkralle. Morph. Jahrb. Bd. 
 
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 KERBERT, C. Ueber die Haut der Reptilien und anderer Wirbelthiere. Arch. 
 
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 OPPENHEIMER, E. Ueber eigentiiml. Organe in d. Haut. einiger Reptilien. Ein 
 
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 OSAWA, GAKUTARO. Beitrage zur feineren Structur des Integuments der Hatteria 
 
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APPENDIX 403 
 
 (d) AVES. 
 
 DAVIES, H. R. Zur Entwicklung der Feder und ihre Beziehungen zu anderen 
 
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 FICALBI, E. Sulla architettura istologica di alcuni peli degli uccelli con con- 
 
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 Vol. XI. 1890. 
 GARDINER, E. Beitr. zur Kenntnis des Epitrichiums und der Bildung des 
 
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 HAECKER, V. Ueber die Farben der Vogelfedecn. Arch. f. mikr. Anat. Bd. 
 
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 MEIJERE, T. C. H. de Ueber die Federn der Vogel, insbesondere uber ihre 
 
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 STUDER, Th. Die Entwicklung der Federn. Inaug. -Diss. Bern, 1873. 
 
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 (e) MAMMALIA. 
 
 ALSHEIMER. A. Ueber die Ohrenschmalzdriisen. Inaug. -Dissert. Wiirzburg,. 
 
 1888. 
 BARDELEBEX, K. v. Ueber 600 neue Falle von Hyperthelie bei Mannern. Verh. 
 
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 BLANCHARD, R. Sur un cas de polymastie et sur la signification des mamelles 
 
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 1885. 
 BLASCHKO, A. Beitrage zur Anatomic der Oberhaut. Arch. f. mikr. Anat. 
 
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 BOAS, J. E. V. Ein Beitrag zur Morphologic der Nagel, Krallen, Hufe und 
 
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 1894 
 BONNET, R. Haarspiralen und Haarspindeln. Morph. Jahrb. Bd. XI. 
 
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 CREIGHTOX, C. On the Development of the Mamma, etc. Journ. Anat. and 
 
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 EMERY, C. Ueber das Verhaltnis der Saugethierhaare zu schuppenartigen 
 
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 ESCHRICHT. Ueber die Richtung der Haare am menschlichen Korper. Arch. f. 
 
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 EXNER, A. Die Function der Menschlicheu Haare. Biol. Centralbl. Bd. 
 
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 FEIERTAG, F. Ueber die Bildung der Haare. Inaug. -Diss. Dorpat, 1875. 
 FJELSTRUP, A. Ueber den Bau der Haut bei Globiocephalus melas. Zool. Anz. 
 
 XL Jahrg. 1888. 
 FLEMMING, W. Ein Drillingshaar mit gemeinsamer innerer Wurzelscheide. 
 
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 GEGENBAUR, C. Zur genaueren Kenntnis der Zitzen der Saugethiere. Morph. 
 
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 Zur Morphologic des Nagels. Morph. Jahrb. Bd. X. 1885. 
 Zur Kenntnis der Mammarorgane der Monotremen. Leipzig, 1886. 
 
 D D 2 
 
404 APPENDIX 
 
 GRAFF, K. Vergl. -anatom. Unters. iiber den Bau der Hautdriisen der Haus- 
 
 saugethiere und des Menschen. Inaug.-Diss. Leipzig, 1879. 
 GROTE, G. Ueber die Glandulae anales des Kaninchens. Inaug. -Dissert. 
 
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 HAACKE, W. Eierlegende Saugethiere. Humboldt. VI. Jahrg. Stuttgart, 
 
 1887. 
 On the Marsupial ovum, mammary pouch and the male milk glands of 
 
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 HESSLING, Th. v. Ueber die Brunftfeige der Gemse. Zeitschr. f. wiss. Zool. 
 
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 Huss, M. Beitrage zur Entw. der Milchclriisen, etc. Jen. Zeitschr. Bd. VII. 
 JAKOBIS. Pathogenese der Pigmentirungen und Entfarbungen der Haut. 
 
 Centralblatb f. allg. Pathol. u. pathol. Anat. I. Bd. No. '20. 1890. 
 KALLIUS, E. Ein Fall von Milchleiste bei einem menschl. embryo. Anat. 
 
 Heft, I. Abth. Heft 24 (Bd. VIII., Heft 1), 1897. 
 KEIBEL, F. Zur Ontogenie u. Phylogenie von Haar u. Feder. Anat. Hefte, 
 
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 KLAATSCH, H. Zur Morphologic der Saugethierzitzen. Morph. Jahrb. Bd. IX 
 
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 Zur Morphologic der Tastballen der Saugethiere. Morph. Jahrb. Bd. XIV. 
 
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 Ueber die Beziehungen zwischen Mammartasche und Marsupium. Morph. 
 
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 Ueber Marsupialrudimente bei Placentaliern. Morph. Jahrb. Bd. XX. 
 
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 Studien zur Geschichte der Mamarorgane. Teil I. Die Taschen- und 
 Beutelbildungen am Driisenfeld der Monotremen. Jena (Semon's Zoolog. 
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 KOLLIKER, A. v. Ueber die Entwicklung der Nagel. Sitz. -Ber. Ges. Wiirzburg, 
 
 1888. 
 KROMAYER. Oberhautpigment der Saugethiere. Arch. f. mikr. Anat. XLIL 
 
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 KUKENTHAL, W. Ueber die Anpassung von Saugethieren an das Leben im 
 
 Wasser. Zool. Jahrbiicher, V. Bd. 1890. 
 Ueber Reste eines Hautpanzers bei Zahnwalen. Anat. Anz. V. Jahrg. 
 
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 LEBOUCQ, H. Rech. sur la Morphologic de la main chez les Mammiferes marins, 
 
 etc. Arch, de Biol. T. IX. 1889. 
 
 LEICHTENSTERN. Ueber iiberzahlige Briiste. Arch. f. path. Anat. 1878. 
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 LIST, J. Ueber die Herkunft des Pigmentes in der Oberhaut. Biol. Centralbl. 
 
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 MARTIN, C. T. and TIDSWELL, FRANK. Observ. on the femoral Gland of Orni- 
 
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 MAURER, F. Haut-Sinnesorgane, Feder- und Haaranlagen, und deren gegenseitige 
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 MEIJERE, T. C. H. de. Ueber die Haare der Saugethiere, besonders tiber ihre 
 
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 NORNER, C. Ueber den feineren Bau des Pferdehufes. Arch. f. mikr. Anat. 
 
 Bd. XXVIII. 1886. 
 
 POULTON, E. B. The structure of the Bill and Hairs of Ornithorhynchus 
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 RAUBER, A. Ueber den Ursprung der Milch und die Ernahrung der Frucht im 
 
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 REIN, G. Unters. iiber d. embr. Entw. -Geschichte der Milchdriise. Arch. f. 
 
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APPENDIX 405 
 
 SCHULTZE, O. Ueber die erste Anlage des Milchdriisenapparates Anat. Anz. 
 
 VII. Jahrg. 1892. 
 Milchdrtisenentwicklung und Polymastie. Sitz.-ber. Ges. Wiirzburg. VIII. 
 
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 8CHWALBE, G. Ueber den Farbenwechsel winterweisser Thiere. Ein Beitrag 
 
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 SILVIO TORRI, G. Sul Significato di un'appendice epiteliale dei follicoli piliferi 
 
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 1896. 
 
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 UNNA, P. Beitr. zur Histologie und Entw. -Geschichte der menschlichen Oberhaut 
 
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 WALDEYER, W. Atlas der menschl. u. thierischen Haare sowie der ahnlichen 
 
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 WEBER, M. Ueber neue Hautsecrete bei Saugethieren. Arch. f. mikr. Anat. 
 
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 Bemerk. iiber den Ursprung der Haare und iiber die Schuppen bei Sauge- 
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 B. EXOSKELETON. 
 
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406 APPENDIX 
 
 THE FOLLOWING DEAL WITH THE EXOSKELETON OF FOSSIL FlSHES, A.Ml'HIHIANs 
 
 AND REPTILES. 
 
 CREDNER, H. (Seep. 396.) 
 
 DOLLO. Numerous papers on Fossil Reptiles in the Bulletin du Musee royal 
 
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 FRAAS, O. Aetosaurus ferratus, Fr. Die gepanzerte Vogelechse aus dem Stuben- 
 
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 FRITSCH, A. Die Reptilien und Fische der bohmischen Kreideformation. Prag, 
 
 1878. 
 Fauna der Gaskohle und der Kalksteine der Permformation Bohmens. Prag, 
 
 187985. 
 
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 MEYER, H. v. Numerous papers in Palaeontographica, e.g. Bd. VI. Archego- 
 
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 ROMER, F. Ueber den Bau, und die Entwicklung des Panzers der Giirtelthiere. 
 
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 RUTIMEYER, L. Ueber den Bau von Schale und Schadel bei lebenden und foss. 
 
 Schildkroten. Verh. Ges. Basel, VI, 1. 
 
 C. ENDOSKELETOK 
 1. VERTEBRAL COLUMN. 
 
 (a) AMPHIOXUS, PISCES, AND DIPNOI. 
 
 AGASSIZ, A. (See p. 395. ) 
 
 CALBERLA, E. Ueber die Entw. d. Medullarrohres und der Chorda dorsalis der 
 
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 CARTIER, 0. Beitr. z. Entw-Geschichte der Wirbelsaule. Zeitschr. f. wiss. Zool. 
 
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 GLAUS, C. Ueber die Herkunft der die Chordascheide der Haie begrenzenden 
 
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 EBNER, V. v. Ueber den feineren Bau der Chorda dorsalis der Cyclostomen. 
 
 Sitz.-ber. Ak. Wien. Bd. C. IV. Abth. III. 1895. 
 Ueber den feineren Bau der Chorda dorsalis von Myxine nebst weiteren 
 
 Bemerkungen iiber die Chorda von Ammocretes. Sitz.-ber. Ak. Wien. 
 
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 1. Ueber den feiner. Ban der Chorda dorsalis v. Acipenser. Loc. cit. 
 
 2. Ueber den ,, ,, ,, ,, ,, ,, bei Amphioxus lanceolatus. 
 Ueb. die Wirbel der Knockenfische und die Chorda dorsalis der Fische u. 
 
 Amphibien. Sitz.-ber. Ak. Wien. Bd. CV. Abth. III. 1896. 
 frEGENBAUR, C. Ueber das Skeletgewebe der Cyclostomen. Jen. Zeitschr. 
 
 Bd. V. 
 Ueber die Entw. der Wirbelsaule des Lepidosteus mit vergl. anat. 
 
 Bemerkungen. Loc. cit. Bd. III. 
 GOTTE, A. Beitrage zur vergl. Morphologic des Skeletsystems der Wirbelthiere. 
 
 Arch. f. mikr. Anat. Bd. XV. 1878. 
 GRASSI, B. Beitr. z. naheren Kenntnis der Entwicklung der Wirbelsiiule der 
 
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 Lo Sviluppo della Colonna vertebrale ne' Pesci ossei. Atti Ace. Lincei, 
 
 188283. 
 GADOW HANS and ABBOTT, E. C. On the Evolution of the Vertebral Column in 
 
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 HASSE, C. Die fossilen Wirbel. Morph. Jahrb. II. (1876), III. (1877), IV. 
 
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 Die Entwicklung der Wirbelsaule der Elasmobranchier. Zeitschr. f. wiss. 
 
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 Die Entwicklung und der Bau der Wirbelsaule der Ganoiden. Loc. cit. 
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APPENDIX 407 
 
 HASSE, C. Die Entwicklung cler Wirbelsaule der Dipnoi. Zeitscbr. f . wiss. Zool. 
 
 Bd. LV. 4. 
 JOSEPH, H. Ueber das Achsenskelet des Amphioxus. Zeitsch. f. wiss. Zool. 
 
 Bd. LIX. 3. 1895. 
 KLAATSCH, H. Beitr. zur vergl. Anatomie der Wirbelsaule. Morph. Jahrb. 
 
 Bd. XIX, XX, XXII, XXIII. 
 KOLLIKER, A. Ueber die Beziehung der Chorda zur Bildung der Wirbel der 
 
 Selachier und einiger anderer Fische. Verh. Ges. Wiirzburg. Bd. X. 
 Weitere Beobachtungen iiber die Wirbel der Selachier. Abh. Senckenb. 
 
 Ges. Bd. V. 
 Ueber das Ende der Wirbelsaule der lebenden Teleostier und einiger Ganoiden. 
 
 Gratul.-Schrift f. d. Univ. Basel 1860. 
 LVOFF, B. Vergl-anatom. Studien iiber die Chorda und die Chordascheide. 
 
 Bull. Soc. Moscou. 1887. 
 Ueber Bau u. Entw. d. Chorda von Amphioxus. Mitth. Zool. Stat. Neapel. 
 
 Bd. IX. 1890. 
 Die Bildung der primaren Keimblatter und die Entstehung der Chorda und 
 
 des Mesoderms bei d. Wirbelthieren. Bull, de Moscou, 1894. 
 MAYER, P. (See p. 420). 
 MULLER, AUG. Beobacht. z. vergl. Anat. der Wirbelsaule. Arch. f. Anat. und 
 
 Physiol. 1853. 
 
 MULLER, W. Ueber den Bau der Chorda dorsalis. Jen. Zeitschr. 1871. 
 RETZIUS G. Uber das hintere Ende des Riickenmarks bei Amphioxus, Myxine 
 
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 SCHEEL, C. Beitr. z. Entw. Gesch. der Teleostierwirbelsaule. Morph. Jahrb. 
 
 Bd. XX. 1893. 
 SCHMIDT, V. Das Schwanzende der Chorda dorsalis bei den Wirbelthieren. 
 
 Anat. Hefte, I Abthiel, VI/VII Heft (II Bd. Heft III/IV). 
 SCHMIDT, L. Untersuch. z. Kenntnis des Wirbelbaues von Amia calva. Inaug. 
 
 Dissert. Strassburg i/E. 1892. 
 WIEDERSHEIM, R. Das Skelet und Nervensystem von Lepidosiren annectens 
 
 (Protopterus). Morphol. Studien. Heft. I. Jena 1880. 
 
 (6) AMPHIBIA, REPTILIA, AND AVES. 
 
 ALBRECHT, P. Ueber einen Processus odontoides des Atlas bei den urodelen 
 
 Amphibien. Centralbl. f. die medic. Wissenschaft. 1878. 
 Ueber den Proatlas, einen zwischen dem Occipitale und dem Atlas der amnioten 
 Wirbelthiere gelegenen Wirbel, und den Nervus spinalis I s. proatlanticus. 
 Zool. Anz. Bd. III. 1880. 
 
 Note sur la presence d'un rudiment de Proatlas sur un exemplaire de Hatteria 
 
 punctata. Extr. Bull. Mus. roy d'hist. nat. de Belgique. Tome II. 1883. 
 
 Notes sur une hemivertebre gauche surnumeraire de Python sebae. loc. cit. 
 
 Tome II. 1883. 
 
 Note sur le basi-occipital des batraciens anoures. loc. cit. Tome II. 1883. 
 BAUR, G. Ueber den Proatlas emer Schildkrote (Platypeltis spinifer Les.). Anat. 
 
 Anz. X. Bd. 1895. 
 Osteolog. Notizen iiber Reptilien. Zool. Anz. IX und X. Jahrg. 1886, 
 
 1887. 
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 VI. Bd. 1886. 
 BLESSIG, E. Eine morphol. Untersuchung iiber die Halswirbelsaule der 
 
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 CLAUS, C. Beitr. z. vergl. Osteologie der Vertebraten. Sitz.-ber. Ak. Wien. 
 
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 COPE, E. D. Extinct Batrachia from the Perm. Form, of Texas. Pal. Bullet 
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 DOLLO, L. Sur la Morphologic de la Colonne vertebrale. Travaux du Laboratoire 
 
 de Wimereux. 1892. 
 EBNER, V. v. (See p. 406. ) 
 
 FRAISSE, P. Beitr. zur Anatomic des Pleurodeles Waltlii. Arb. Inst. Wiirzburg, 
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408 APPENDIX 
 
 FBAISSE, P. Eigenthiimliche Structurverhaltnisse im Schwanz erwachsener 
 
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 FIELD, H. H. Bemerk. iiber die Entwicklg. d. Wirbelsaule der Amphibien etc. 
 
 Morph. Jahrb. Bd. XXII. 1895. 
 GADOW, H. On the Evolution of the vertebral column of Amphibia and Amniota. 
 
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 GEGENBAUR, C. Unters. z. vergl. Anatomic der Wirbelsiiule der Amphibien u. 
 
 Reptilien. Leipzig 1862. 
 GOTTE, A. Ueber die Zusammensetzung der Wirbelsaule bei den Reptilien. 
 
 Zool. Anz. 1894. 
 Ueber den Wirbelbau bei den Reptilien und einigen andern Wirbel thieren. 
 
 Zeitschr. f. wiss. Zool. Bd. LXII. 1896. 
 HASSE, C. Anatomische und palaontologische Ergebnisse. Leipzig 1878. 
 
 Die Eiitwicklung der Wirbelsaule von Triton tseniatus. Zeitschr. f. wiss. 
 
 Zool. L. III. Bd. Suppl. 1892. 
 
 Die Entwicklung der Wirbelsaule der ungeschwanzten Amphibien. Loc. cit. 
 HASSE C. und SCHWARCK. Stud, zu vergl. Anat. d. Wirbelsiiule etc. in "Hasse, 
 
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 HOFFMANN, C. K. Beitr. z. vergl. Anatomic d. Wirbelthiere. Niederl. Arch. f. 
 
 Zool. Bd. IV., V. 
 MARSH, O. C. Various articles on fossil Reptiles and Birds. Amer. Journ. of 
 
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 MULLER, E. Uber die Abstossung u. Regeneration des Eidechsensthwanz.es. Jahr. 
 
 Ber. d. Vereins fiir vaterland. Naturkunde Wiirtemburg. 52 Jahrgang 
 
 Stuttgart. 1896. 
 
 PARKER, W. K. On the Morphology of Birds. P. Roy. Soc. Vol 42. 1887. 
 PETER, K. Die Wirbelsaule der Gymnophionen. Ber. Ges. Freiburg. Bd. IX. 
 
 1894. 
 
 Ueber die Bedeutung des Atlas der Amphibien. Anat. Anz. Bd. X. 1895. 
 RIDEWOOD, W. G. On the development of the vertebral column in Pipa and 
 
 Xenopus. Anat, Anz. Bd. XIII. 1887. 
 SCHWINK, F. Ueber die Entwicklung des mittleren Keimblattes und der Chorda 
 
 dorsalis der Amphibien. Miinchen, 1889. 
 SIEBENROCK, F. Zur Kenntnis des Rumpfskeletes der Scincoiden, Anguiden und 
 
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 1895. 
 VAILLANT, LEON. Mem. sur la disposition des vertebres cervicales des Cheloniens. 
 
 Ann. sci. nat. zool. art. No. VII. 
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 ZITTEL, K. Ueber Flugsaurier aus dem lithograph. Schiefer Bayerns. Palaeonto- 
 
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 MAMMALIA. 
 
 ALBRECHT, P. Die Epiphysen und die Amphiomphalie der Saugethierwirbelkorper. 
 
 Zool. Anz. 1879. 
 Note sur un sixieme costoide cervical chez un jeune Hippopotamus amphibius. 
 
 Extr. du Bull. Mus. roy. d'hist. nat. de Belgique. Tome I. 1882. 
 Ueber die Wirbelkdrpergelenke zwischen dem Epistropheus, Atlas und 
 Occipitale der Saugethiere. Comptes rendus des internat. medic. Congresses. 
 Kopenhagen 1884. 
 
 Ueber die Chorda dorsalis und sieben knocherne Wirbelcentren im knorpeligen 
 Nasenseptum eines erwachsenen Rindes. Biol. Centralbl. Bd. V. No. 
 5u. 6. 
 CORNET, J. Note sur le pretendu Pro-Atlas des Mammiferes et de Hatteri 
 
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 CUNNINGHAM, D.J. The Lumbar Curve in Man and the Apes, with an account of 
 the topogr. Anatomy of the Chimpanzee, Orang-Utang, and Gibbon. Tr. 
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 EBNER, V. v. Urwirbel und Umgliederung der Wirbelsaule. Sitz.-ber. Ak. 
 
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 Ueber die Beziehungen der Wirbel zu den Urwirbeln. Loc. cit. Bd. C. I. 
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APPENDIX 409 
 
 ECKER, A. Der Steisshaarwirbel, die Steissbeinglatze und das Steissbeingriibchen, 
 
 etc. Arch. f. Anthropologie. Bd. XII. 
 
 FOL, H. Sur la queue de 1'embryon humain. Comptes rendus. 1885. 
 FRORIEP, A. Zur Eiitwickl. -Geschichte der Wirbelsaule, insbesondere des Atlas 
 
 und Epistropheus und der Occipital-Region. Arch. f. Anat. u. Physiol. 
 
 1886. 
 GERLACH, L. Em Fall von Schwanzbildung bei einem menschl. Embryo. Morph. 
 
 Jahrb. Bd. VI. 
 HASSE, C. und SCHWARCK. Studien zur vergl. Anatomic der Wirbelsaule etc. 
 
 Hasse, Anat. Studien. Heft I. 
 KEIBEL, F. Ueber die Entwicklungsgeschichte der Chorda bei Saugern (Meer- 
 
 schweinchen und Kaninchen). Arch. f. Anat. 1889. 
 Ueber den Schwanz des menschlichen Embryos. Arch. Anat. 1891. (See 
 
 also Anat. Anz. VI. Jahrg. 1891.) 
 KOLLJKER, A. Ueber die Chordahohle und die Bildung der Chorda beim 
 
 Kaninchen. Sitz. her. Ges. Wiirzburg. 1883. 
 LEBOTJCQ, H. Recherches s. 1. mode de Disparition de la corde dorsale chez les 
 
 Vertebres superieurs. Arch. Biol. Vol. I. 1880. 
 ROSENBERG, C. Ueber die Entwicklung der Wirbelsaule und das Centrale Carpi 
 
 des Menschen. Morph. Jahrb. Bd. I. 1876. 
 ROSENBERG, E. Ueber die Wirbelsaule der Myrmecophaga jubata. Festschrift 
 
 fiir Carl Gegenbaur. Leipzig, 1896. 
 STEINBACH, E. Die Zahl der Caudalwirbel beim Menschen. Inaug. -Dissert 
 
 Berlin. 1889. 
 WALDEYER, W. Die Caudalanhange des Menschen. Sitz.-ber. Ak. Berlin. 
 
 1896. 
 
 2. RIBS AND STERNUM. 
 
 ALBRECHT, P. Ueber die im Lauf der phylogenetischen Entwicklung entstandene^ 
 
 angeborene Spalte des Brustbeinhandgriffes des Briillaffen. Sitz.-ber. Ak. 
 
 Berlin. XX. 1885. 
 BARDELEBEN, K. Ueber das Episternum des Menschen. Sitz.-ber. Jenaisch 
 
 Gesellsch. f. Medic, u. Naturwiss. 1879. 
 BAUR, G. On the Morphology of Ribs. Amer. Nat. 1887. 
 
 On the Morphology of Ribs, etc. Journ. Morphol. Vol. III. 1889. 
 Ueber Rippen und rippenahnliche Gebilde und deren Nomenclatur. Anat. 
 
 Anz. IX. Bd., 1893. 
 BLANCHARD, R. La septieme cote cervicale de I'homme. Revue scientif. 1885. 
 
 (3e serie.) No. 23. 
 CLAUS, C. Beitrage zur vergl. Osteologie der Vertebraten. Sitz.-ber. Ak. Wien. 
 
 Bd. LXXIV. 1876. 
 DOLLO, L. Sur la Morphologic des Cotes. Bull. sci. France-Belgique, Tome 
 
 XXIV. 1892. 
 EHLERS, E. Zoolog. Miscellen. I. Der Processus Xiphoideus und seine Muscu- 
 
 latur von Manis macrura, Erxl, und Manis tricuspis, Sundev. Abh. Ges. 
 
 Gottingen. Bd. 39. 1894. 
 FICK, A. E. Zur Entw. -Geschichte der Rippen und Querfortsiitze. Arch. f. 
 
 Anat. und Physiol. 1879. 
 GEGENBAUR, C. Ueber die epistern. Skelettheile und ihr Vorkommen bei den 
 
 Saugethieren und beim Menschen. Jen. Zeitschr. Bd. I. 
 GOPPERT, E. Zur Kenntnis der Amphibienrippen. Zool. Jahrb. XXII. Bd. 
 
 1895. 
 
 Untersuchungen zur Morpholog. d. Fischrippen. Loc. cit. Bd. XXIII. 
 Die Morphologic der Amphibienrippen. Festschrift fiir Carl Gegenbaur. 
 
 Leipzig, 1896. 
 GOTTE, A. Beitrage zur vergl. Morphologic des Skeletsystems der Wirbelthiere. 
 
 Arch. f. mikr. Anat. Bd. XV., pag. 143147. (See also p. 397.) 
 Beitrage zur vergl. Morphologic des Skeletsystems der Wirbelthiere, 
 
 Brustbein und Schultergiirtel. Arch. f. mik. Anat. Bd. XIV. 
 HASSE, C., und BORN, G. Bemerkungen iiber die Morphologic der Rippen. Zool. 
 
 Anz. 1879. 
 H \SWELL, W. A. Studies on the Elasmobranch Skeleton. P. Linn. Soc. N.S.W 
 
 Vol. IX. 1884. 
 
410 APPENDIX 
 
 HATSCHEK. Die Rippen der Wirbelthiere. Verh. Anat. Ges. 1889. Jena, 1889. 
 
 HOFFMANN, C. K. (See p. 408.) 
 
 HOWES, G. B. The Morphology of the Sternum. Reprinted, with a Correction, 
 
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 LEBOTJCQ, H. Rech. sur les variations anatomiques de la premiere cote, chez 
 
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 LINDSAY, B. On the avian Sternum. P. Zool. Soc. 1885. 
 MULLER, A. Beitr. z. vergl. Anat. der Wirbelsaule. Miiller's Archiv. 1853. 
 PARKER, W. K. A monograph on the structure and development of the shoulder- 
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 PARKER, T. J. On the Origin of the Sternum. Tr. N.Z. Inst. Vol. XXIII. 
 
 1890. 
 On the Presence of a Sternum in Notidanus indicus. "Nature," Vol. 43, 
 
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 PILLING, E. Ueber die Halsrippen des Menschen. Inaug. Dissert. Rostock. 1894. 
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 1892. 
 RATHKE, H. Ueber den Bau und die Entwicklung des Brustbeins der Saurier. 
 
 Konigsberg 1853. 
 RUGE, G. Untersuchungen iiber Entwicklungsvorgange am Brustbeine und an 
 
 der Sternoclavicularverbindung des Menschen. Morph. Jahrb. Bd. VI. 
 
 1880. 
 
 SIEBENROCK, F. (See p. 408.) 
 WHITE, P. J. A Sternum in Hexanchus griseus. Anat. Anz. XI. Bd. No. F. 
 
 1895. 
 WIEDERSHEIM, R. (See p. 415 and 416.) 
 
 3. SKULL. 
 
 (a) PISCES. 
 
 AHLBORN, F. Ueber die Segmentation des Wirbelthierkorpers. Zeitschr. f. wiss. 
 
 Zool. 1884. 
 BAUR, G. On the Morphology and Origin of the Ichthyopterygia. Amer. Nat. 
 
 1887. 
 BRIDGE, T. W. On certain features of the skull of Osteoglossum formosum. P. 
 
 Zool. Soc. 1895. 
 DOHRN, A. Studien zur Urgeschichte des Wirbelthierkorpers. Mittheil. Zool. 
 
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 Bemerk. iiber den neuesten Versuch einer Losung des Wirbelthierkopf- 
 
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 DOLLO, L. Nouvelle Note sur le Champsosaure, Rhynchocephalien adapte a la vie 
 
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 FOOTE, Ethelwyn. The extra-branchial cartilages of the Elasmobranchs. Anat. 
 
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 GADOW, H. On the Modifications of the first and second visceral arches with 
 
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 GEGENBAUR, C. Unters. z. vergl. Anat. d. Wirbelthiere. III. H. Das Kopskelet 
 
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 Ueber das Kopfskelet von Alecocephalus rostratus, Risso. Festgabe des 
 
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 Ueber die Occipitalregion und die ihr benachbarten Wirbel der Fische. Fest- 
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 Die Metamerie des Kopfes und die Wirbeltheorie des Kcpfskeletcs. Morph. 
 
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 HASWELL, W. A. (Seep. 409.) 
 
APPENDIX 411 
 
 HATSCHEK. Die Metamerie des Amphioxus und des Ammocoetes.' Verh. Anat, 
 
 Ges. 1892. 
 HOFFMANN, C. K. Zur Entwickl. Gesch. des Selachierkopfes. Anat, Anz. 
 
 IX. Bd. 1894. 
 HUXLEY, T. H. The nature of the craniofacial apparatus of Petromyzon. Journ. 
 
 Anat. and Physiol. Vol. X. 
 JAEKEL, O. Uber die Organisation der Pleuracanthiden. Sitz.-ber. Naturf. 
 
 Berlin, Jahrg. 9. 1895. 
 
 KILLIAN, G. Zur Metamerie des Selachierkopfes. Verh. Anat. Ges. 1891. 
 KLEIN, v. Beitr. zur Bildung des Schadels der Knochenfisehe. Jahresb. des 
 
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 KUPFFER, C. Studien z. vergl. Entw. -Gesch. d. Kranioten. I. Heft. Die Ent- 
 
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 Miinchen u. Leipzig, 1893. II. Heft. Die Entwicklung des Kopfes von 
 
 Petromyzon planeri, 1894. III. Heft. Die Entwicklung des Kopfnerven 
 
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 Entwicklungsgeschichte des Kopfes. Anat. Hefte Ergebn. 1895. 
 LOGY, W. A. Contrib. to the Structure and Development of the vertebrate Head. 
 
 Journ. Morphol. Vol. XL, No. 3. 1895. 
 MARSHALL, A. MILNES. The segmental value of the cranial nerves. Journ. Anat. 
 
 and Physiol. Vol. XVI. 
 On the Head Cavities and Associated Nerves in Elasmobranchs. Q. Journ. 
 
 Micr. Science. Vol. XXI. 
 PARKER,, W. K. On the Structure and Development of the Skull in Sharks and 
 
 Skates. Tr. Zool. Soc. Vol. X., Pt. IV. 
 On the Skeleton of the Marsipobranch Fishes. Part L, The Myxinoids 
 
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 On the Structure and Development of the Skull in the Sturgeons. Phil. 
 
 Trans. 1882. 
 On the Development of the Skull in Lepidosteus osseus. Phil. Trans. 
 
 1882. 
 On the Structure and Development of the Skull in the Salmon. Phil. Trans. 
 
 Vol. CLXIIL, 1873. 
 PARKER, W. K., und BETTANY, G. T. The Morphology of the Skull. London, 
 
 1877. (German Trans, by B. Vetter. Stuttgart, 1879.) 
 PLATT, J. A Contribution to the Morphology of the Vertebrate Head. Journ. 
 
 Morphol. Vol. V., 1891. 
 Further Contributions to the Morphology of the Vertebrate Head. Anat. 
 
 Anz. VI. Jahrg., 1891. 
 POLLARD, H. B. The Suspension of the Jaws in Fish. Anat. Anz. X. Bd. No. 1, 
 
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 The Oral Cirri of Siluroids and the Origin of the Head in Vertebrates. Zool. 
 
 Jahrb. Bd. VIII. , 1895. 
 POUCHET, D. Du developpement du squelette des poissons osseux. Journ. de 
 
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 RABL. Ueber die Metamerie des Wirbelthierkopfes. Verh. Anat. Ges., 1892. 
 RIDEWOOD, W. G. On the Spiracle and Associated Structures in Elasmobranch 
 
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 ROSENBERG, E. Unters iiber die Occipitalregion des Cranium und den proximalen 
 
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 Ueber das Kopfskelet einiger Selachier. Sitz. ber. der Dorpater Natur- 
 
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 SAGEMEHL, M. Beitrage zur vergl. Anat. der Fische. Morph. Jahrb. Bd. IX. 
 
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 Beitrage zur vergl. Anatomic der Fische. IV. Das Cranium der Cyprinoi- 
 
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 SEWERTZOFF, A. Die Entwicklung der Occipitalregion der niederen Vertebral en 
 
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 VROLIK, J. A. Studien iiber die Verknocherung und die Knochen des Schadcls 
 
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412 APPENDIX 
 
 WALTHER, J. Die Entwicklung der Deckknochen m Kopfskelet des Hechtes. 
 
 Jen. Zeitschr. XVI. N. F. IX. 
 WHITE, P. J. The Existence of Skeletal Elements between the Manclibular and 
 
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 1895. 
 WIJHE, J. W. VAN. Ueber das Visceral skelet und die Nerven des Kopfes der 
 
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 Ueber die Mesodermsegmente und die Entwickelung der Nerven des 
 
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 WRIGHT, RAMSAY R. On the Skull and Auditory Organ of the Siluroid 
 
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 (6) DIPNOI. 
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 (r) AMPHIBIA. 
 
 BORN, G. Ueber die Nasenhohlen und den Thranennasengang der Amphibien. 
 
 Bresl. Habil.-Schrift. Leipzig, 1877, and Morphol. Jahrb. Bd. II., 
 
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 Beitr. z. Morphol. d. Schadels. III. Zur vergl. Anatomic oier Schlafengegend 
 
 etc. Morph. Arb. IV. Bd. 1. H., 1894. 
 HAY, O. P. The Skeletal Anatomy of Amphiuma during its Earlier Stages. 
 
 Journ. Morphol. Vol. IV. No. 1, 1890. 
 HERTWIG, O. Ueber das Zahnsystem der Amphibien und seine Bedeutung f iir 
 
 die Genese des Skelets der Mundhohle. Arch. f. mikr. Anat. Vol. XI. 
 
 Suppl.-H., 1874. 
 HUXLEY, T. H. On the Structure of the Skull and of the Heart of Menobraiichus 
 
 lateralis. P. Zool. Soc., 1874, pt. II. 
 
 KINGSLEY, J. S. The Head of an Embryo Amphiuma. Amer. Nat., 1892. 
 PARKER, W. K. On the Structure and Development of the Skull in the Urodelous 
 
 Amphibia, pt. I. Phil. Trans. London, 1877. 
 On the Morphology of the Skull in the Amphibia Urodela. Tr. Linn. Soc. 
 
 (Ser. 2) Zool. Vol. II. 
 On the Structure and Development of the Skull in the Urodeles. Tr. Zool. 
 
 Soc., 1882. 
 On the Structure and Development of the Skull of the common Frog. Phil. 
 
 Trans. London, 1871. 
 On the Structure and Development of the Skull in the Batrachia, pt. II. 
 
 Phil. Trans. 1881. 
 On the Structure and Development of the Skull in the Batrachia, pt. III. 
 
 Phil. Trans., 1881. 
 REICHSBT, C. B. Vergl. Entw.-Geschichte des Kopfes der nackten Amphibien. 
 
 Konigsberg, 1838. 
 K i I:SE, H. Beitr. z. Anatomic des Tylototriton verrucosus. Zool. Jahrb. 
 
 Bd. V., 1891. 
 
APPENDIX 413 
 
 SCHULZE, F. E. Ueber die inneren Kiemen der Batrachierlarven etc. I. II. 
 
 Abth. Ab. Ak. Berlin, 1888 und 1892. 
 STOHR, PH. Zur Entw. -Geschichte des Urodelenschadels. Wiirzburger Habil. - 
 
 Schrift, 1879, and Zeitschr. f. wiss. Zool. Bd. XXXIII. 
 Zur Entw. -Geschichte des Anurenschadels. Zeitschr. f. wiss. Zool. Bd. 
 
 XXXVI. 
 WALTER, F. Das Visceralskelet und seine Muskulatur bei den einheimischen 
 
 Amphibien und Reptilien (gekr. Preisschrift). Jen. Zeitschr. Bd. XXI. 
 
 N. F. XVI. 
 WIEDERSHEIM, R. Das Kopfskelet der Urodelen. Morph. Jahrb. Bd. III. 
 
 1877. 
 Das Skelet von Pleurodeles Waltlii. Morph. Studien. Heft 1. (See also 
 
 page 397.) 
 
 (d) REPTILIA. 
 
 BAUR, G. Osteolog. Notizen iiber Reptilien. ZooL.Anz. Jahrg. IX. 1886. (See 
 
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 BEMMELEX, J. F. VON. Bemerk. zur Phylogenie der Schildkroten. Compte-rendu 
 
 d. seances du troisieme Congres internat. de Zool. Leyde, 1896. 
 Bemerkungen iiber den Schadelbau von Dermochelys coriacea. Festschrift 
 
 fiir Carl Gegenbaur. Leipzig, 1896. 
 GAUPP, E. Beitr zur Morphol. d. Schiidels. III. Fiir vergl. Anat. der Schlafen- 
 
 gegend, &c. Morph. Arb. IV. Bd. J. H., 1894. 
 OPPEL, A. Ueber Vorderkopfsomiten und die Kopfhohle von Anguis fragilis. 
 
 Arch. f. mikr. Anat. Bd. XXXVI. 1890. 
 PARKER, W. K. On the structure and development of the skull in the common 
 
 Snake. Phil. Trans. 1878. 
 On the structure and development of the skull in the Lacertilia. Phil. Trans. 
 
 1879. 
 The development of the Green Turtle. The Zoology of the Voyage of H.M.S. 
 
 Challenger. Vol. I., pt. V. 
 On the structure and development of the skull in the Crocodilia. Tr. Zool. 
 
 Soc. Vol. XIX. pt. IX. 1883. 
 
 On the structure of the skull in the Chameleons. Tr. Zool. Soc., 1881. 
 uSiEBENROCK, F. Zur Keiintnis des Kopfskeletes der Scincoiden, Anguiden und 
 Gerrhosauriden. Annal. d. K. K. Naturhistor. Hof museums. Bd. VII. 
 Heft 3. Wien, 1892. (See also p. 408.) 
 
 (e} AVES. 
 
 PARKER, W. K. On the structure and development of the skull of (a) the Ostrich 
 
 tribe ; Phil. Trans., 1866 ; (b) the common Fowl. Phil. Trans., 1869. 
 On (Egithognathous birds. Parts I. and II. Tr. Zool. Soc. Vols. IX. and X. 
 On the morphology of the skull in the Woodpeckers and Wrynecks. Tr. 
 
 Linn. Soc. (ser. 2) Zool., vol. I., 1875. 
 On the structure and development of the Bird's Skull, part II., loc. cit. (see 
 
 also various papers in Monthly Micros. Journ., 1871-1873). 
 PARKER, T. J. On the skeleton of Notornis mantelli. Trans. N. Y. Inst. 
 
 Vol. XIV. 1881. (See also Vol. XVIII. 1885.) 
 On the classification and mutual arrangement of the Dinornithidse. Loc. cit. 
 
 Vol. XXV. 1892. 
 On the cranial osteology, classification and phylogeny of the Dinornithidse. 
 
 Zool. Soc. Vol. XIII. 1895. 
 WALKER, M. L. On the form of the quadrate bone in Birds. Stud. Mus. of 
 
 Zool. in University College, Dundee. 1888. 
 (Comp. also p. 398. ) 
 
 (/) MAMMALIA. 
 
 ALBRECHT, P. Sur la fente maxillaire double sousmuqueuse et les 4 os inter- 
 
 maxillaires de 1'Ornithorhynche adulte normal. Bruxelles, 1883. 
 Memoire sur le basiotique, un nouvel os de la base du crane, situe entre 
 1'occipital et le sphenoide, presente a la Societe Pathologique de Bruxelles. 
 Bruxelles, 1883. 
 
414 APPENDIX 
 
 ALBRECHT, P. Sur les 4 os intermaxillaires, le bec-de-lievre et la valeur morpho- 
 
 logique des dents incisives superieures de 1'homme. Bruxelles, 1883. 
 Sur la valeur morphologique de 1'articulation mandibulaire, du cartilage de 
 
 Meckel et des osselets de 1'ou'ie avec essai de prouver que 1'ecaille du tem- 
 poral des mammiferes est coinposee primitivement d'un squamosal et d'un 
 
 quadratum. Bruxelles, 1883. 
 ALLEN, HARRISON. On a revision of the ethmoid bone in the Mammalia. Bull. 
 
 Mus. Harvard. Vol. X. No. 3. 
 BAUR, G. Ueber das Qiiadratimi der Saugethiere. Biol. Centralbl. Bd. VI. 1887, 
 
 uiitl Gesellsch. fiir Morphol. mid Physiol. zu Miinchen. 1886. 
 COLLINGE, W. E. The skull of the Dog. A manual for students. London and 
 
 Birmingham, 1896. 
 
 COPE. E. D. The phylogeny of the Camelidae. Amer. Nat. Extra, July, 1886. 
 DECKER, F. Ueber den Primordialschadel einiger Saugethiere. Zeitschr. f. wiss. 
 
 Zool. Bd. XXXVIII. 1884. 
 DUBOIS, EUG. Pithecanthropus erectus, eine menschenahnl. Ubergangsform. 
 
 Batavia, 1894. (See also Aiiat. Anz., Bd. XII.) 
 DURSY, E. Entw. -Geschichte des Kopfes des Menschen und der hoheren 
 
 Wirbelthiere. Tiibingen, 1869. 
 FIOALEI, E. Sulla ossificazione delle Capsule periotiche nell'uomo e negli altri 
 
 Mammiferi. Atti d. R. Accadem. Medica di Roma. Vol. III. Ser. II, 
 
 1886-87. 
 FRORIEP, A. Bemerkungen zur Frage nach der Wirbeltheorie des Kopfskeletes. 
 
 Anat. Anz. II. Jahrg. 1887. 
 
 HALLMANN. Die vergl. Anatomic des Schlafenbeins. 1837. 
 HARTLAUB, C. Beitrage zur Kenntnis der Manatus-Arten. Zool. Jahrb. Bd. I. 
 
 1886. 
 
 HOWES, G. B. On the mammalian hyoid. Report Brit. Assoc. Adv. Sci., 1895. 
 JACOBY, M. Ein Beitrag zur Kenntnis des menschlichen Primordialcraniums. 
 
 Arch. f. mikr. Anat. Bd. 44. 1894. 
 JOSEPH, G. Morphol. Studieii am Kopfskelet des Menschen und der Wirbelthiere. 
 
 Breslau, 1873. 
 LATASTE, F. Les Cornes des Mammiferes. Dans leur axe osseux aussi bien que 
 
 dans leur revetement corne sont des productions cutanees. Act. Soc. Scient. 
 
 du Chili, IV. annee. T. IV. 5 livr. 1894. 
 
 LUCAE, J. Die Sutura transversa squamae occipitis. Abh. Senkenb. Ges. 
 MIHALKOVICS, V. VON. (See under Olfactory Organ.) 
 NATHUSIUS, H. v. Vorstudien fiir Geschichte und Zucht der Hausthiere zunachst 
 
 am Schweineschadel. Mit einem Atlas. 1864. 
 NEUNER, R. Ueber angebliche Chordareste in der Nasencheidewand des Rindes. 
 
 Inaug. -Dissert. Miinchen, 1886. 
 OSBORN, H. F. See numerous papers on Bulletin of the American Museum of 
 
 Natural History. (Comp. also Wortman, loc. cit.) 
 PARKER, W. K. On the structure and development of the skull in the Pig. Phil. 
 
 Trans. 1874. 
 On the structure and development of the skull in the Mammalia. Part II. 
 
 Edentata, Phil. Trans. No. 232, 1884; and part III., Insectivora, loc. cit. 
 
 No. 235. 1885. 
 RUTIMEYER, L. Versuch einer naturlichen Geschichte des Rindes, &c. Neue 
 
 Denkschr. d. Allg. schweiz. Gesellsch. f. d. ges. Naturw. 22 Bd. 1866. 
 Die Rinder der Tertiarepoche, etc. Abh. Schweiz. pala3ont. Gesellsch. Bd. 
 
 IV. 1877. 
 
 Ueber das zahme Schwein und das Hausrind. Verh. Ges. Basel. VII. 1. 1882. 
 Beitrage z. Gesch. d. Hirschfamilie. I. Schadelbau. Verh. Ges. Basel. 
 
 VI. 3. 1877. 
 SALKNSKY, W. Beitr. z. Entw. -Geschichte der knorpeligen Gehorknochelchen 
 
 bei Saugethieren. Morph. Jahrb., Bd. VI. 
 
 SCHWINK, F. Ueber den Zwischenkiefer und seine Nachbarorgane bei Sauge- 
 thieren. Miinchen, 1888. 
 STONDLI, H. Ueber den Primordialschadel der Saugethiere und des Menschen. 
 
 Inaug. -Dissert. Zurich, 1846. 
 STEHLIN, H. G. Fur Kenntnis der postembryonalen Schadelmetamorphosen bei 
 
 Wietlerkauern. Basel, 1893. 
 
APPENDIX 415 
 
 4. PECTORAL ARCH. 
 
 DOIIKX, A. Studien zur Urgeschichte des Wirbelthierkorpers. VI. Die paarigen 
 und unpaaren Flossen der Selachier. Mittheil. Zool. Stat. Neapel. V. 
 Band, 1 Heft. 1886. 
 
 DUGES. Recherches sur 1'osteologie et la myologie des Batraciens a leurs different 
 
 ages. Memoires presentes par divers savants a I'academie royale des 
 
 sciences de 1'institut de France. Sciences mathematiques et physiques. 
 
 Tom VI. Paris, 1835. 
 
 EMKKY, C. et SIMONI, L. Recherches sur la ceinture scapulaire des Cyprinoides. 
 
 Arch, ital Biol. T. VII., Fasc. 3. 1886. 
 Ueber die Beziehungen des Cheiropterygium zum Ichthyopterygium. Zool. 
 
 Anz., X. Jahrg. 1887. 
 GARMAN, S. Chlamydoselachus anguineus, Garm. A living species of cladodont 
 
 Shark. Bull. Mus. Harvard. 
 
 GEGENBAUR, C. Uiiters. zur vergl. Anatomic der Wirbelthiere : Schultergiirtel der 
 Wirbelthiere. Carpus und Tarsus und Brustflosse der Fische. Leipzig, 
 1864-65. 
 
 Clavicula und Cleithrum. Morph. Jahrb. Bd. XXIII. 1895. 
 GOLDI, E. A. Kopfskelet und Schultergiirtel von Loricaria cataphracta, Balistes 
 capriscus und Acipenser ruthenus. Vergl. anatomisch-entwicklungs- 
 geschichtl. Studien /ur Deckknochenfrage. Jena, 1884. 
 HOFFMANN, C. K. Beitrage zur vergl. Anatomic der Wirbelthiere. Niederland. 
 
 Archiv f. Zool. Vol. V. 1879. 
 Zur Morphologic des Schultergiirtels und des Brustbeines bei Reptilien r 
 
 Vogeln, Saugethieren und dem Menschen. Loc. cit. 
 HOWES, G. B. Observations on the pectoral fin skeleton of the living Batoid 
 
 fishes and of the extinct genus Squaloraja, &c. P. Zool. Soc. 1890. 
 The Morphology of the Mammalian Coracoid. Journ. Anat. and Physiol. 
 
 Vol. XXI. (N. S. Vol. I.) 1887. 
 
 On the Coracoid of the Terrestrial Vertebrata. P. Zool. Soc. June 20, 1893. 
 HYRTL, J. (Seep. 397.) 
 PARKER, W. K. A monograph on the structure and development of the Shoulder 
 
 Girdle and Sternum in the Vertebrata. Ray Society. 1868. 
 SABATIER, A. Comparaison des ceintures et des membres anterieurs et posterieurs 
 
 dans la Serie des Vertebres. Montpellier, 1880. 
 SWIRSKI, G. Unters. iiber die Entwicklung des Schultergiirtels und des Skelets 
 
 der Brustflosse des Hechtes. Inaug. -Dissert. Dorpat, 1880. 
 WIEDERSHEIM, R. Morphologische Studien. Heft I. Jena, 1880. 
 
 Das Gliedmassenskelet der Wirbelthiere mit besonderer Beriicksichtigung 
 des Schulter-und Beckengiirtels bei Fischen, Amphibien und Reptilien. 
 Jena, 1892. (See also pp. 397 and 407.) 
 
 5. PELVIC ARCH. 
 
 ALBRECHT, P. Note sur le pelvisternum des Edentes. Bruxelles, 1883. 
 
 BAUR, G. The Pelvis of the Testudinata ; with notes on the evolution of the 
 
 Pelvis in general. Journ. Morphol. Bd. IV. Boston, 1891. 
 BOLK, L. Beziehungen zwischen Skelet, Musculatur und Nerven der Extremi- 
 
 taten, dargelegt am Beckengiirtel, an desseu Musculatur und am Plexus 
 
 lumbo-sacralis. Morph. Jahrb., Bd. XXI. 1894. 
 BUNGE, A. Unters. zur Entwicklung des Beckengiirtels der Amphibien, 
 
 Reptilien und Vogel. Inaug. -Dissert. Dorpat, 1880. 
 DOHRN, A. (See above. ) 
 GEGENBAUR, C. Ueber den Ausschluss des Schambeines von der Pfanne de 
 
 Hiiftgelenkes. Morph. Jahrb., Bd. II. 1876. 
 
 Beitr. z. Kenntnis des Beckens der Vogel. Jen. Zeitschr., Bd. VI. 1871. 
 GORSKI. Ueber das Beck en der Saurier. Inaug. -Dissert. Dorpat, 1852. 
 HILGENDORF, R. Das Ileo-Sacral-Gelenk der zungenlosen Frosche (Pipa, Dacty- 
 
 lethra). Sitz. ber. Naturf. Berlin, 1884. 
 HOFFMANN, C. K. Beitr. z. Kenntnis des Beckens der Amphibien und Reptilien. 
 
 Niederl. Arch. f. Zool., Bd. III. 
 
416 APPENDIX 
 
 HOWES, G. B. On the Mammalian Pelvis with especial Reference to the young 
 
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 HUXLEY, T. H. On the characters of the pelvis in the Mammalia, &c. P. Roy. 
 
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 JOHNSON, ALICE. On the DeA*elopment of the pelvic Girdle and Skeleton of the 
 
 Hind Limb in the Chick. Stud. Morph. Lab. Camb. Vol. II., parti. 1884. 
 
 LECHE, W. Das Vorkommen und die morphol. Bedeutnng des Pfannenknocheiis 
 
 (Os acetabuli). Internationale Monatsschrift fiir Anatomic und Histologie. 
 
 Bd. I. 1884. 
 
 Zur Morphologic der Beutelknochen. Verhcllg. d. Biolog. Vereins zu 
 Stockholm. III. Bd. 1891. No. 7. Comp. numerous articles on Fossil 
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 MEHNEBT, E. Untersuch. iiber die Entwicklung des Os pelvis der Vogel. 
 
 Morph. Jahrb., Bd. XIII. 1888. 
 Untersuch. iiber die Entwicklung des Beckengiirtels bei einigen Saugethieren. 
 
 Loc, cit. Bd. XV. 1889. 
 Untersuch. iiber die Entwicklung des Beckengiirtels der Emvs lutaria 
 
 taurica. Loc. cit. Bd. XVI. 1790. 
 
 Untersuch. iiber -die Entwicklung des Os hypoischium (Os cloacae), Os 
 epipubis und Ligamentum medianum pelvis bei den Eidechsen. Loc. cit. 
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 SABATIER M. A. Compar. d. ceintures thoraciques et pelv. dans la Serie des 
 
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 WIEDEBSHEIM. R. Ueber das Becken der Fische. Morph. Jahrb., Bd. VII. 
 
 1881. 
 
 Zur Urgeschichte des Beckens. Ber. Ges. Freiburg, VI. 1888. 
 Ueber die Entwicklung des Schulter- und Beckengiirtels. Anat. Anz. V. 
 
 Jahrg. 1894. No. 1. 
 Weitere Mittheilhungen iiber die Entwicklungsgeschichte des Schulter- und 
 
 Beckengiirtels. Anat. Anz., V. Jahrg. 1890 No 1. 
 
 Die Phylogenie der Beutelknochen. Eine entwicklungsgeschichtlich- 
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 WOODWARD, A. SMITH. Palaeontological Contributions to Selachian Morphology. 
 P. Zool. Soc. 1888. 
 
 6. FINS AND LIMBS (free portion). 
 
 ALBRECHT, P. Beitrag zur Torsionstheorie des Humerus und zur morph. 
 
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 Kiel, 1875. 
 Das Os intermedium tarsi der Saugethiere. Zool. Anz., VI. Jahrg. No. 
 
 145. 1883. 
 1) Os trigone du pied chez l'homme, 2) Epihallux chez 1'homme. 3) Epiphyses 
 
 entre 1'occipital et le sphenoide chez l'homme. Bulletin de la Societe 
 
 d'anthropologie de Bruxelles. Tome III., 3e fascicule. 1885. 
 BALFOUR, F. M. On the development of the skeleton of the paired fins of 
 
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 BARDELEBEN, K. Das Os intermedium tarsi der Saugethiere. Zool. Anz., VI. 
 
 Jahrg. 1883. No. 139. 
 Beitr. z. Morphologic des Hand- und Fusskeletes. Sitz.-ber. d. Jen. 
 
 Gesellsch. Medic, u. Naturwissensch. 1885. 
 Ueber neue Bestandtheile der Hand- und Fusswurzel der Saugethiere, 
 
 sowie das Vorkommen von Rudimenten " iiberzahliger " Finger und 
 
 Zehen beim Menschen. Jen. Zeitschr., Bd. XIX. N. F. XII. Suppl. 
 
 Heft III. 1886. 
 On the Bones and Muscles of the Mammalian Hand and Foot. P. Zool. Soc. 
 
 1894. 
 
 Hand und Fuss, Referat Verh. Anat. Ges., Jahrg. 8. 1894. 
 BAUR, G. Der Tarsus der Vogel und Dinosaurier. Morph. Jahrb., Bd. VIII. 
 
 1883. 
 Ueber das Archipterygium und die Entwicklung des Cheiropterygium. Zool. 
 
 Anz. No. 209, VIII. Jahrg. 1885. 
 
APPENDIX 417 
 
 BAUR, G. Zur Morphologie cles Carpus und Tarsus der Reptilien. Loc. cit. No. 
 
 208. VIII. Jahrg. 1885. 
 Bemerkungen iiber den Astragalus und das Intermedium tarsi der Sauge- 
 
 thiere. Morph. Jahrb., Bd. XL 1885. 
 Zur Morphologie des Carpus und Tarsus der Wirbelthiere. Zool. Anz. , VIII. 
 
 Jahrg. 1885. 
 
 Der alteste Tarsus (Archegosaurus). Loc. cit. Jahrg. IX. 1886. 
 Die zwei Centralia im Carpus von Sphenodon (Hatteria) und die Wirbel von 
 
 Sphenodon und Gecko verticillatus Laur. (G. verus Gray.) Zool. Anz. 
 
 Jahrg. IX. 1886. 
 
 Ueber die Kanale im Humerus der Amiiioten. Morph. Jahrb. Bd. XII. 1887. 
 Beitrage zur Morphologie des Carpus und Tarsus der Vertebraten. I. Theil : 
 
 Batrachia. Jena, 1888. 
 Neue Beitr. z. Morphol. des Carpus der Saugethiere. Anat. Anz. Jahrg. 
 
 IV. 1889. 
 
 BEMMELEN, VAN. Ueber die Herkunft der Extremitaten- und Zungenmuskulatuv 
 
 bei den Eidechsen. Anat. Anz. Jarg. IV. 1889. 
 BOAS, J. E. V. a) Ueber den Metatarsus der Wiederkauer. b) Ein Fall von 
 
 vollstandiger Ausbildung des 2 und 5 Metacarpale beim Rind. Morph. 
 
 Jahrb. Bd. XVI. 1890. 
 BORN, G. Die sechste Zehe der Anuren. Morph. Jahrb. Bd. I. 1876. 
 
 Eine frei hervorragende Anlage der vorderen Extremitat bei Embryonen von 
 
 Anguis fragilis. Zool. Anz. 1883, No. 150. 
 Ueber das Skelet des Fersenhockers von Rana fusca, &c. Sitz. d. Schles. 
 
 Gesellsch. f. vaterland. Kultur voni 2 Juli 1879. 
 
 Zum Carpus und Tarsus der Saurier. Morph. Jahrb. Bd. II. 1876. 
 Nachtrage zuin Carpus und Tarsus. Morph. Jahrb. Bd. VI. 1880. 
 BOYER, E. R. The mesoderm in Teleosts, especiallv its share in the formation 
 
 of the pectoral fin. Bull. Mus. Harvard. Vol. XXIII. No. 2. 1892. 
 BRANDT, E. Vergl. anatom. Untersuchungen iiber die Griffelbeine (Ossa calami- 
 
 formia) der Wiederkauer. Zool. Anz. XI. 1888. 
 BRIDGE, T. W. The mesial fins of Ganoids and Teleosts. Journ. Linn. Soc. 
 
 Zool. Vol. XXV. 1896. 
 BUNGE, A. Ueber die Nachweisbarkeit eines biserialen Archipterygiums bei 
 
 Selachiern und Dipnoern. Jen. Zeitschr. Bd. VIII. 
 CARLSSON, A. Von den weichen Theilen des sogen. Praepollex und Praehallux. 
 
 Biolog. Foreningens Forhandlingar (Verh. d. biolog. Vereins) in Stockholm. 
 
 Stockholm, 1890. 
 Untersuch. iiber die weichen Theile der sog. iiberzaligen Strahlen an Hand 
 
 und Fuss. K. Svenska. Vet.-Akad. Handlinger. Bd. XVI. Afd. 4. 
 
 Stockholm, 1891. 
 COPE, E. D. New and little known Paleozoic and Mezozoic Fishes. Journ. Acad. 
 
 Nat. Sci. Philad. 2nd Ser. Vol. IX. 
 
 CUENOD, A. L'articulation du coude. Int. Monatschr. Anat. 1888. Bd. V. 
 DAVIDOFF, M. v. Beitr. z. vergl. Anatomic der hinteren Gliedmassen der Fische. 
 
 Morph. Jahrb. Bd. V., VI, IX. 
 DEAN, BASHFORD. The Fin -fold origin of the paired limbs in the light of the 
 
 Ptychopterygia of Palaeozoic Sharks. Anat. Anz. Bd. XL 1896. (See 
 
 also Nat. Sci. Vol. VIII. 1896.) 
 
 DODERLEIN, L. Das Skelet von Pleuracanthus. Zool. Anz. Jahrg. XII. 1889. 
 DOHRN, A. Studien zur Urgeschichte des Wirbelthierkorpers. VI. Die paarigen 
 
 und unpaarigen Flosseii der Selachier. Mittheil. Zool. Stat. Neapel. Bd. 
 
 V. Heft 1, 1886. 
 
 DOLLO, L. Sur 1'origine de la nageoire caudale des Ichthyosaures. Bull. Soc. 
 
 Beige Geol. T. VI. 1892. 
 
 DUCRET, E. Contrib. a 1'etude du developpement des membres pairs et impairs 
 des poissons teleosteens, type Trutta lacustris. Inaug. Dissert. Lausanne, 
 1894. 
 
 EISLER, P. Die Homologie der Extremitaten. Abh. Ges. Halle. Bd. XIX. 1895. 
 EMERY, C. Ueber die Beziehungen cles Cheiropterygiums zum Ichthyoptery- 
 
 gium. Zool. Anz. Jahrg. X. 1887. 
 
 Zur Morphologie des Hand- und Fussskelets. Anat. Anz. Jahrg. V. 1890. 
 Ulteriori studi sullo scheletro della mano degti Anfibi Anuri. Atti. Ace. 
 (Rend.)Lincei. Vol I, 1. Serie 5a. 1892. 
 
 E E 
 
418 APPENDIX 
 
 EMERY, C. Studi sulla Morfologia del Membri del Mammiferi. Mem. R. Accad.- 
 
 d. Scienze dell'Istituto di Bologna. T. II. 1892. 
 
 Studi sulla Morfologia dei Membri degli Anfibi sulla Filogenia del Chiropteri- 
 gio. Ricerche Lab. Anat. Roma e altri. Lab. Biologici. Vol. IV. Fasc. 1. 
 1894. 
 SullaMorfologia.de! Tarso dei Mammiferi. Atti. (Rend.) Ace. Lincei. Vol. 
 
 IV. 2o sem., Ser. 5a, fasc. llo. 1895. 
 Beitr. z. Ent. Gesch. u. Morphol. des Hand. u. Fuss. -Skelets der Marsupi- 
 
 alier. Semon's Forschungsreisen, &c. Bd II 1897. 
 
 EWART, J. C. The Development of the Skeleton of the Limbs of the Horse. 
 Journ. of Comp. Pathology and Therapeutics. 1894. (Comp. also Journ. 
 Anat. and Physiol. Vol. XXVIII. 1894. ) 
 FRAAS, E. Ueber einen neuen Fund von Ichthyosaurus in Wiirttemberg. Neues 
 
 Jahrb. f. Mineralogie, &c. Bd. II. 1892. 
 FRITSCH, A. Ueber die Brustflosse von Xenacanthus Decheni, Goldf. Zool. Anz. 
 
 Bd. XI. 1888. 
 
 Fauna der Gaskohle und der Kalksteine der Permformation Bohmens. Prag, 
 18791890. Bd. III. Heft 1. Selachii (Pleuracanthus, Xenacanthus), 
 Prag, 1890. 
 FURBRINGER, M. Die Knochen und Muskeln der Extremitaten bei den 
 
 schlangenahnlichen Sauriern. Leipzig, 1870. 
 
 Untersuchungen zur Morphologic und Systematik der Vogel, zugleich ein 
 Beitrag zur Anatomic der Stiitz- und Bewegungsorgane. II. Theile. 
 Amsterdam, 1888. 
 Ueber die Nervencanale im Humerus der Amnioten. Morph. Jahrb. Bd. 
 
 XI. 1886. 
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 HARRISON, R. G. Ueber die Entwicklung der nicht knorpelig vorgebildesen 
 
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APPENDIX 419 
 
 HENKE, W. und REYHER, C. Studien iiber die Entwicklung der Extremitaten des 
 
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 HOFFMAXX, C. K. (Seep. 415.) Niederl. Arch. f. Zool. Bd. IX. 
 HOLL, M. Ueber die Entwicklung der Stellung der Gliedmassen des Menschen. 
 
 Sitz.-ber. Ak. Wien. Bd. C. Abth. 3. Febr. 1891. 
 
 HOWES, G. B. On the Skeleton and Affinities of the Paired Fins of Ceratodus y 
 
 With Observations upon those of the Elasmobranchii. P. Zool. Soc. 1887. 
 
 On the pedal skeleton of the Dorking Fowl, with remarks on hexadacty- 
 
 lism and phalangeal variation. Journ. Anat. and Physiol. Vol. XXVI 
 
 1892. 
 
 Notes on variation and development of the vertebral and limb skeleton of the 
 
 Amphibia. P. Zool. Soc. 1893 (and see p. 415). 
 
 HOWES, G. B. and DA VIES, A. M. Observations upon the Morphology and 
 Genesis of Supernumerary Phalanges, with especial reference to those of 
 the Amphibia. P. Zool. Soc. 1888. 
 HOWES, G. B. and RIDEWOOD, R. On the Carpus and Tarsus of the Anura. P. 
 
 Zool. Soc. 1888. 
 
 Hi MPHRY, G. M. Observations on the limbs of vertebrate animals ; the plan of 
 their construction ; their homology and the comparison of the fore and 
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 HUXLEY, T. H. On Ceratodus. (See p. 395. ) 
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 JUXGERSEX, HECTOR F. E. Remarks on the structure of the Hand in Pipa 
 
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 KEHRER. G. Beitr. zur Kenntnis des Carpus und Tarsus der Amphibien, 
 
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 KLAATSCH, H. Die Brustflosse der Crossopterygier. Ein Beitrag zur Anwen- 
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 Festschrift fiir Carl Gegenbaur. Leipzig, 1896. 
 
 KOLLMANX, J. Handskelet und Hyperdactylie. Verb. Anat. Ges. 1888. 
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 Mittheilungen iiber den Carpus' des Weisswals. Morph. Jahrb. Bd. XIX. 
 
 1892. 
 
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 1882. 
 Resume d'un memoire sur la morphologic du carpe chez les mammiferes. 
 
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 II. Jahrg. 1887. 
 L'Apophyse stylo'ide du 3e Metacarpien chez l'homme. Annal. de la Soc. 
 
 de Medecine de Gand. 1887. 
 
 Rech. sur la Morphologic de la main chez les Pinnipedes. Studies from the 
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 Rech. sur la Morphologic de la main chez les Mammiferes marins (Pinnipedes, 
 
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 LEIGHTOX, V. L. The development of the wing of Sterna Wilsonii. Amer. 
 
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 E E 2 
 
420 APPENDIX 
 
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 Arts, Vol. XVI. XXIII. The following are of special interest : 
 
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 2) The wings of Pterodactyles (Vol. XXIII.). 
 
 3) Polydactyle horses, recent and extinct (Vol. XVII.). 
 
 4) On the Affinities and Classification of the Dinosaurian Reptiles (Vol. L. ). 
 
 5) Restoration of some European Dinosaurs (loc. cit.) 
 
 Recent polydactyle horses. Amer. Journ. of Science. Vol. XLIII. April, 
 
 1892. 
 MARTINS, CH. Nouvelle comparaison des membres pelviens et thoraciques chez 
 
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 On the Fate of the Muscle Plate, and the Development of the Spinal 
 
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APPENDIX 421 
 
 ROSENBERG, E. Ueber einige Entwicklungsstadien des Handskelets der Emys 
 
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422 APPENDIX 
 
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 BRACKET, A. Rech. sur le developp. du Diaphragme et du Foie chez le lapin. 
 
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 BROOKS, H. On the Morphology of the Extensor Muscles. Studies from the 
 
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 BRUNER, H. L. Ein neuer Muskelapparat zum Schliessen und Oeffnen der 
 
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 DUGES, A. Rech. sur 1'osteologie et la myologie des batraciens a leurs difterents 
 
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 FURBRINGER, M. Die Knochen und Muskeln der Extremitaten bei den schlan- 
 
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 GADOW, H. Unters. iiber die Bauchmuskeln der Krokodile, Eidechsen und Schild- 
 
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 GiGLio-Tos, E. SulP Omologia tra il Diaframma degli Anfibi aruri e quello dei 
 
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 GORSKI, v. Ueber das Becken der Saurier. Inaug.-Diss. Dorpat, 1852. 
 GRENACHER. Muskulatur der Cyclostomen und Leptocardier. Zeitschr. f. wiss. 
 
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 HAIR, M. On the muscular Fibres of the Alligator. Journ. Anat. and Physiol. 
 
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 HARRISON, R. G. The Metamerism of the Dorsal and the Ventral Longitudinal 
 
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APPENDIX 423 
 
 HAUGHTOX, M. On the muscular Anatomy of the Crocodile. P. Irish Ac., 
 
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 HEPBURN, D. The Comparative Anatomy of the Muscles and Nerves of the 
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 His, W. Mitbheil. zur Embryologie der Saugethiere und des Menschen. Arch. 
 
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 HOLL, M. Zur Homologie und Phylogenese der Muskeln des Beckenausganges 
 
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 HUMPHRY, G. M. The muscles of the smooth Dog-Fish (Mustelus leevis). Journ. 
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 The muscles of Ceradotus. Loc. cit. 
 
 The muscles and nerves of the Cryptobranchus japonicus. Loc. cit. 
 The muscles of Lepidosiren annectens with the cranial nerves. Loc. cit. 
 The muscles of the Glass-Snake (Pseudopus Pallasii). Loc. cit. 
 On the disposition of muscles in vertebrate animals. Loc. cit. 
 On the disposition and homologies of the extensor and flexor muscles of the 
 
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 KAESTNER, S. Ueber die allgemeine Entwicklung der Rumpf- und Schwanz- 
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 Die Entwicklung der Extremitaten-und Bauchmuskulatur bei den anuren 
 
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 XILLIAN, G. Zur vergl. Anatomic und Entwicklungsgeschichte der Ohrmuskeln. 
 
 Anat. Anz. V. Jahrg. 1890. 
 
 KOHLBRUGGE, J. H. F. Versuch einer Anatomic des Genus Hylobates (Muskeln 
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 LARTSCHNEIDER, T. Die Steissbeinmuskeln des Menschen. Denk. Ak. Wien. 
 
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 LECHE, W. Zur Morphologic der Beckenregion der Insectivora. (Vorl. Mitthlg.) 
 
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 MACALISTER, A. On the homologies of the flexor muscles of the vertebrate 
 
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 MAN, DE. Verglijkende myologische en iieurologische Studien over Amphibien 
 
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 MAURER, F. Der Auf ban und die Entwicklung der ventralen Rumpfmuskulatur bei 
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 Selachier und Teleostier. Morph. Jahrb. Bd. XVIII. 1892. 
 Die Elemente der Rumpfmuskulatur bei Cyclostomen und hoheren Wirbel- 
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 MIVART, ST. G. On the myology of Menopoma allegh., Menobranchus lat., and 
 
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 PERRIN, A. Contrib. a 1' etude de la myologie comparee : Membre posterieur 
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 POPOWSKY, T. Zur Entwicklungsgeschichte des N. facialis beim Menschen. 
 
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 RAVN, E. Unters. iiber die Entwicklung des Diaphragmas und der benachb. Organe 
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 RETZIUS, G. Biol. Untersuclmngen. N. F. I. No. 2. Muskelfibrille und 
 
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 REX, H. Ein Beitrag zur Kenntnis der Muskulatur der Mundspalte der Affen. 
 
 Morph. Jahrb. Bd. XII. 1887. 
 RUGE, G. Entwickl. -Vorgange an der Muskulatur des menschl. Fusses. Morph. 
 
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424 APPENDIX 
 
 RUGE, G. Untersuchungen iiber die Extensorengruppe am Unterschenkel mid Fus^ 
 
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 Ueber die Gesichtsmuskulatur der HalbafFen. Morph. Jahrb. Bd. XI. 1885. 
 Untersuchungen iiber die Gesichtsmuskulatur der Primaten. Leipzig, 1887. 
 Die vom Facialis innervirten Muskeln des Halses, Nackeris und des Schadels 
 
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 1887. 
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 E. ELECTRIC ORGANS. 
 
 BABUCHIN. Ueber die Bedeutung und Entwicklung der pseudoelektrischen 
 
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 Entwicklung der elektrischen Organe und Bedeutung der mot. Endplatten. 
 
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 Uebersicht derneueren Untersuchungen iiber Entwicklung, Ban, und physiol. 
 
 Verhaltnisse der elektrischen und pseudo-elektrischen Organe. Arch. f. 
 
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 Beobachtungen und Versuche am Zitterwelse und Mormyrus des Niles. Arch. 
 
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 BALLOWITZ, E. Ueber den Ban des elektrischen Organes von Torpedo mil 
 
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APPENDIX 425 
 
 BALLOWITZ, E. Ueber den feineren Ban des elektrischen Organs des gewohn- 
 
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 BOLL, F. Beitrage z. Physiologic von Torpedo. Arch. f. Anat. und Physiol. 
 
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 (1) die Structur der elektrischen Flatten von Torpedo. (2) Die Structur der 
 
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 Neue Untersuchungen zur Anat. und Physiol. von Torpedo. Monatsbericht 
 
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 Neue Untersuchungen iiber die elektrischen Platten von Torpedo. Arch. f. 
 
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 Nerven-physik. Bd. II. 
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 F. NERVOUS SYSTEM. 
 
 (a) CENTRAL NERVOUS SYSTEM. 
 
 1. PISCES. 
 
 AHLBORN, F. Zur Neurologic der Petromyzonten. (Vorl. Mitth.) Nachr. Ges. 
 
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 Untersuchungen iiber das Gehirn der Petromyzonten. Zeitsch. f. wiss. Zool. 
 
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 AUERBACH, L. Die Lobi optici der Teleostier und die Vierhiigel der holier 
 
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426 APPENDIX 
 
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 BURCKHARDT, R. Zur vergl. Anat. des Vorderhirns bei Fischen. Anat. Anz. 
 
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 Ueber einige bemerkenswerthe Elemente des Centralnervensy stems von 
 
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APPENDIX 427 
 
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 NANSEN, F. The Structure and Combination of the Histological Elements of the 
 
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 Anat. u. Physiol. 1882. 
 Entwicklung des Knochenfischgehirns (Entw. derZirbel). Sitz.-ber. Naturf. 
 
 Berlin. 1882. 
 Weiteres zur Deutung des Gehirns der Knochenfische. Biol. Centralbl. 
 
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 Das Grosshirn der Knochenfische und seine Anhangsgebilde. Arch. f. Anat. 
 
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 Das Gehirn der Knochenfische. Vortrag, gehalten in der Gesellschaf t fur 
 
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 Zur onto- und phylogenetischen Entwicklung des Torus longitudinalis im 
 
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 Der Lobus olfactorius impar der Selachier. Anat. Anz. VIII Jahrg 
 
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 Das Vorderhirn der Cranioten. Eine Antwort an Herrn F. K. Studnicka. 
 
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428 APPENDIX 
 
 
 
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 2. DIPNOI. 
 
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 3. AMPHIBIA. 
 
 BURCKHARDT, R. Untersuchungen am Hirn und Geruschorgan von Triton und 
 
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APPENDIX 429 
 
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 Observations upon the Presence of the Corpus Callosum in the Brains of 
 
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 1891. 
 
 4. REPTILIA. 
 
 BRAUX, M. Die Entwicklung des Wellenpapageies, etc. Wu'rzburg. 1881, and 
 
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 Entw. - Vorgange am Schwanzende bei einigen Saugethieren, etc. Arch. f. 
 
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 HOFFMANN, C. H. Weitere Untersuchungen zur Entw.-Gesch. der Reptilien. 
 
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 OSBORX, H. F. (See above.) 
 RABL-RUCKHARD, H. Das Centralnervensystem des Alligators. Zeitschr. f. 
 
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 Ueber das Vorkommen eines Fornixrudimentes bei Reptilien (Psammosaurus 
 
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 Einiges iiber das Gehirn der Riesenschlange. Zeitschr. f. wiss. Zool. Bd. 
 
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 5. AVES. 
 
 BRAXDIS, F. Untersuchungen iiber das Gehirn der Vogel. Arch. f. mikr. Anat. 
 
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 BUMM, A. Das Grosshirn der Vogel. Zeitschr. f. wiss. Zool., Bd. XXXVIIIL, 
 
 1883. 
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 Zeitschr. f. wiss. Zool., Bd. XIX. 
 
430 APPENDIX 
 
 6. MAMMALIA. 
 
 BEDDAKD, F. E. On the Brain in the Lemurs. P. Zool. Soc., 1895. (Comp. 
 
 also Deridrolagus and Gulo in the same vol. ) 
 BEN HAM, W. BLAXLAND. A Description of the Cerebral Convolutions of the 
 
 Chimpanzee known as "Sally" ; with Notes on the Convolutions of other 
 
 Chimpanzees and of two Orangs. Q. Journ. Micr. Sci., Vol. 37, N.S.. 
 
 1894. 
 BERTELLI, D. II solco intermediario anteriore del midollo spinale umano. Atti 
 
 Soc. Toscana. Memorie, Vol. XI., 1890. 
 BISCHOFF, TH. Die Grosshirnwindungen des Menschen. Miinchen, 1868 (Comp. 
 
 also papers on the Brain of the Chimpanzee, Orang and Gorilla in Sitz.-ber. 
 
 Miinchener Acad. 1874, 1876 and 1877). 
 
 BROCA, P. Etude sur le cerveau du Gorille. Revue d'anthropologie, 1878. 
 Anatomic comparee des circonvolutions cerebrales. Loc. cit., 1878. 
 Rech. sur les centres olfactifs. Loc. cit., 1879. 
 CAJAL, RAMON Y. Neue Darstellung vom Bau des Centralnervensystems. Arch. 
 
 f. Anat., 1893. 
 
 CUNNINGHAM, D. J. Contribution^ the Surface Anatomy of the Cerebral Hemi- 
 spheres, with a chapter upon Cranio-Cerebral Topography by Victor 
 
 Horsley. Tr. Irish Ac., Cunningham Memoirs, No. VII., 1892. 
 DEXTER, F. A Contribution to the Morphology of the Medulla Oblongata of the 
 
 Rabbit. Arch. Anat., 1896. 
 DURSY, E. Zur Entw. -Geschichte des Kopfes des Menschen und der hoheren 
 
 Wirbelthiere. Tubingen, 1869. 
 EBERSTALLER, 0. Das Stirnhirn. Ein Beitrag zur Anatomic der Oberflache des. 
 
 Grosshirns. Wien und Leipzig, 1890. 
 ECKER, A. Zur. Entw.-Gesch. der Furchen und Windungen der Grosshirnhemi- 
 
 sphJiren im Fotus des Menschen. Arch. f. Anthrop., Bd. III. 
 Die Hirnwindungen des Menschen. Braunschweig, 1869. II. Aufl., 1885. 
 EDINGER, L. (Seep. 426.) 
 FAMILIANT, v. Beitrage zur Vergleichung der Hirnfurchen bei den Carnivoren 
 
 und den Primaten, etc. Inaug. -Dissert. Bern, 1885. 
 FISH, P. A. The Indusium of the Callosum. Journ. Comp. Neurology. Vol. 
 
 III., 1893. 
 FI.ESCH, M. Ueber die Hypophyse einiger Saugethiere. Tageblatt der 58 Ver- 
 
 sammlung deutscher Naturforscher u. Aerzte in Strassburg, 1885. (See also^ 
 
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 FLECHSIG, P. Die Leitungsbahnen im Gehirn und Riickenmark des Menschen. 
 
 Auf Grund entw. -geschichtl. Unters. Leipzig, 1876. 
 GANSER, S. Vergl. anat. Studien iiber das Gehirn des Maulwurfs. Morph. 
 
 Jahrb., Bd. VII., 1882. 
 GIACOMINI, C. Sul cervello d'un Chimpanse. Atti. Ace. Torino. Vol. XXIV. , 
 
 1889. 
 GIERKE, H. Die Stiitzsubstanz des Centralnervensystems. Arch. f. mikr. Anat. 
 
 Bd. XXV., 1885, u. Bd. XXVI., 1896. 
 GOLGI, C. Ueber den feineren Bau des Riickenmarkes. Anat. Anz. V. Jahrg. , 
 
 1890. 
 
 Unters. lib. d. fein. Bau d. central, u periph. Nervensystems. Jena, 1894. 
 GULDBERG, G, Zur Morphologic der Insula Reilii. Anat. Anz. II. Jahrg., 1887. 
 His, W. .Zur Geschichte des menschlichen Riickenmarkes und der Nerven- 
 
 wurzeln. Abh. d. math.-phys. Cl. der K. Sachs. Ges. d. Wissensch. 
 
 Bd. XIII. No. VI. Leipzig', 1886. 
 Die Neuroblasten und deren Enstehung im embryonalen Mark. Loc. cit. , 
 
 Bd. XV. Leipzig, 1888. 
 Zur Geschichte des Gehirns sowie der centralen und peripheren Nerven- 
 
 bahnen beim menschlichen Embryo. Loc. cit., Bd. XIV. Nr. VII. 
 
 Leipzig, 1888. 
 
 Die Formentwicklung des menschl. Vorderhirns. Loc. cit., Bd. XV., 1889. 
 Die Entwicklung des menschlichen Rautenhirns, etc. Loc. cit., Bd. XVII. 
 
 1890. 
 Histogenese und Zusammenhang der Nervenelemente. Referat i. d. anatom. 
 
 Section des internat. medizin. Congresses z. Berlin. 1890. (See also 
 
 Arch, fur Anat. und Physiol., Suppl.-Bd. 1890.) 
 
APPENDIX 431 
 
 His, W. Ueber das frontale Ende des Gehirnrohres. Arch, f . Anat. und Physiol. 
 
 1893. 
 
 Vorschliige zur Eintheilung des Gehirns. Loc. cit. 
 
 Ueber die Vorstufen der Gehirn- iind Kopfbildung bei Wirbelthieren. 
 Sonderung und Charakteristik der Entwickl. Stufen junger Selachierem- 
 bryonen. Loc. cit., 1894. 
 JELGERSMA, G. Ueber den Bau des Saugethiergehirns. Morph. Jahrb. XV. 
 
 Bd. 1889. 
 KINGSBURY, B. F. On the Brain of Necturus maculatus. Journ. Comp. Neurol. 
 
 Vol. V. 1895. 
 KOLLIKER, A. Der feinere Bau des verlangerten Markes. Anat. Anz. VI. 
 
 Jahrg. 1891. 
 KRUEG, J. Ueber die Furchung der Grosshirnrinde der Ungulaten. Zeitschr. f. 
 
 wiss. Zool., Bd. XXXI, 1879. 
 Ueber die Furchen auf der Grosshirnrinde der zonoplacentalen Saugethiere. 
 
 Zeitschr. f. wiss. Zool. Bd. XXXIII. 1881. 
 
 KUKENTHAL, W. Vergl. anatom. und entwickl. -geschichtl. Untersuchungen an 
 Walthieren. [Kapitel III : Das Centralnervensystem der Cetaceen ; 
 gemeinsam mit Th. Ziehen.] Jena, 1889. 
 
 KUKENTHAL, W., and ZIEHEX, TH. Untersuchungen iiber die Grosshirnfurchen 
 der Primaten. Jen. Zeitschr, Bd. 29. N. T. 22. (Comp. also Anat. 
 Anz. XL Bd, No. 15.) 
 LENHOSSEK, M. v. Ueber die Pyramidenbahnen im Riickenmark einiger 
 
 Saugethiere. Anat. Anz. IV. Jahrg. No. 7. 1889. 
 Zur Kenntnis der Neuroglia des menschlichen Riickenmarkes. Verh. Anat. 
 
 Ges. 1891. 
 Der feinere Bau des Nervensy stems im Lichte neuester Forschungen. 
 
 Fortschritte der Medizin. Bd. X. 1892. 
 Beitrage zur Histologie des Nervensy stems und der Sinnesorgane. Wiesbaden, 
 
 LEURET ET GRATIOLET. Anatomic comparee du systeme nerveux. Paris, 1839- 
 
 1857. 
 LOTHRINGER, S. Untersuchungen an der Hypophyse einiger Saugethiere und 
 
 des Menschen. Arch. f. mikr. Anat. Bd. XXVIII. 1886. 
 LUGARO, E. Sulla genesi delle Circonv r oluzioni cerebrali e cerebellari. Riv. di 
 
 Pathologia nervosa el mentale. Vol. II., fasc. 3. 1897. 
 LUYS>, J. Recherches sur le systeme nerveux cerebrospinal. Paris, 1865. Mit 
 
 Atlas von 40 Tafeln. 
 
 Iconographie photographique des centres nerveux. Paris, 1872. 
 MARCHAND, F. Ueber die Entwicklung des Balkens im menschl. Gehirn. Arch. 
 
 f. mikr. Anat. Bd. XXXVII. 1891. (Comp. also the paper by L. 
 
 BLUMENAU. Loc. cit.) 
 Die Morphologic des Stirnlappens und der Insel der Anthropomorphen 
 
 Arbeiten a. d. pathol. Inst. z. Marburg. Bd. II. 1893. 
 MARTIN, P. Fur Entwicklung des Gehirnbalkens bei der Katze. Anat. Anz. 
 
 IX. Bd. 1893. 
 MIHALCOVICS, V. v. Entw. -Geschichte des Gehirns. Nach Untersuch. an hoheren 
 
 Wirbelthieren und dem Menschen. Leipzig, 1877. 
 NUSBAUM, J. Einige neue Thatsachen zur Entw. Geschichte der Hypophysis 
 
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 PANSCH, A. De sulcis et gyris in cerebris simiarum et hominum. Kieler Habil.- 
 
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 Ueber die typische Anordnung der Furchen und Windungen auf den Gross- 
 
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 Bd. III. 
 Ueber gleichwerthige Regionen am Grosshirn der Carnivoren und der 
 
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 Beitr. zur Morphologic des Grosshirns der Saugethiere. Morph. Jahrb., Bd. 
 
 V, 1879. (Canina, Felina, Ursina, Mustelina.) 
 
 REICHERT, C. B. Der Bau des menschlichen Gehirns. Leipzig, 1859 und 1861. 
 RETZIUS, G. Ueber ein dem Saccus vasculosus entsprechend. Gebilde am 
 
 Gehirn des Menschen und der Saugetiere. Biol. Untersuch. N. T. VII. 
 
 1895. 
 Das Menschengehirn. Mit Atlas. Stockholm, 1896. 
 
432 APPENDIX 
 
 SCHWALBE, G. Lehrbuch der Neurologie. Zugleich als zweite Abtheilung des 
 
 zweiten Bandes von Hoffmann's Lehrbuch der Anatomic des Menschen. 
 
 Erlangen, 1880. 
 Der Arachnoidalraum, ein Lymphraum und sein Zusammenhang mit dem 
 
 Perichorioidalraum. Medic. Centralbl. 1869. No. 30. 
 SMITH, G. ELLIOT. The Brain of a Foetal Ornithorhynchus. Q. Journ. Micros. 
 
 Sci. Vol. 39', 1896. 
 The Comparative Anatomy of the Cerebrum of Notoryctes typhlops. 
 
 Tr. R. Soc., S. Australia, XIX. (See also Anat. Anz., 1895.) 
 SYMINGTON, J. The cerebral commissures in the Marsupialia and Monotremata. 
 
 Journ. Anat. and Physiol. Bd. XXVII. 1892. 
 The cerebral commissures in the Marsupialia and Monotremata. Journ. 
 
 Anat. and Physiol. Vol. XXVII. 1892. (Comp. also Elliott Smith, P. 
 
 Linn. Soc., N."S.W Vol. IX. 
 TIEDEMANN, Fr. Anatomic und Bildungsgeschichte des Gehirns im Fotus des 
 
 Menschen. Niirnberg, 1816. 
 TURNER, W. The Convolutions of the Brain, etc. Journ. Anat. and Physiol. 
 
 1890. 
 WALDEYER, M. Das Gorilla-Ruckenmark. Abh. Ak. Berlin, 1888. 
 
 Das Gibbonhirn. Festschr. R. Virchow gewidmet. 1891. Internat. Beitr. 
 
 zur wissensch. Medizin. Bd. I. 
 WEBER, W. Vorstudien ueber das Hirngewicht der Saugethiere Zeitschrift fur 
 
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 WILDER, B. G. (See p. 428.) 
 ZIEHEN, TH. Zur vergl. Anat. der Hirnwindungen mit spec. Beriicks. der 
 
 Gehirne von Ursus maritimus und Trichechus rosmarus. Anat. Anz. V. 
 
 Jahrg. No. 24. 1890. 
 Die Grosshirnfurchen des Hylobates und Semnopithcrus-Gehirnes, Loc. cit. 
 
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 ZUCKERKANDL, E. Ueber das Riechcentrum. Eine vergleichend-anatomische 
 
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 (b) PINEAL BODY (INCLUDING FRONTAL AND PARIETAL ORGAN). 
 
 AHLBORN, F. Ueber die Bedeutung der Zirbeldrtise. Zeitschr. f. wiss. Zool. 
 
 Bd. XL. 1884. 
 BEARD, J. The parietal Eye in Fishes. Nature, Vol. 36. 1887. 
 
 Morphol. Studies No. 1. The Parietal Eye of the Cyclostome Fishes. Q. 
 
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 BERANECK, E. Ueber das Parietalauge der Reptilien. Jen. Zeitschr. 1887. 
 Sur le nerf parietal et la morphologic du troisieme ceil des Vertebres. Anat. 
 
 Anz. VII. Jahrg. 1892. 
 
 L'individualite de 1'cuil parietal. Loc. cit. VIII. Jahrg. 1893. 
 Contrib. a 1'embryogenie de la glande pineale des Amphibiens. Revue 
 
 Suisse de Zoologie et Annale du Musee d'hist. nat. de Geneve. T. I. 1893. 
 CARRIERS, J. Neuere Untersuchungen iiber das Parietalorgan. Biol. Centralbl. 
 
 Bd. IX. No. 5. 1889. (Kritische Uebersicht.) 
 
 CATTIE, J. Ueber die Epiphyse der Fische. Arch, de Biol. Vol. III. 1882. 
 DUVAL et KALT. Des yeux pineaux multiples chez 1'Orvet. Soc. de Biologic. 
 
 1889. No. 6. 
 EHLERS, E. Die Epiphyse am Gehirn der Plagiostomen. Zeitschr. f. wiss. Zool. 
 
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 EYCLESHYMER. Paraphysis and Epiphysis in Amblystoma. Anat. Anz. VII. 
 
 Jahrg. 1892. 
 FLESCH, M. Ueber die Deutung der Zirbel bei den Saugethieren. Anat. Anz. 
 
 III. Jahrg. 1888. 
 
 FRANCOTTE. Rech. sur le developpement de 1'epiphyse. Arch, de Biol. 1888. 
 GRAAF, H. DE. Bydrage tot te Kennis van den Bouw en de Ontwikkeling der 
 
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 Zur Anatomic und Entwicklung der Epiphyse bei Amphibien und Reptilien. 
 
 Zool. Anz. IX. Jahrg. 1886. 
 HAMTSCH, R. On the Pineal Eye of the Young and Adult Anguis fragilis. 
 
 Proc. Biol. Soc. Liverpool. Vol. III. 1889. 
 
APPENDIX 433 
 
 HILL, CH. Development of the Epiphysis in Coregonus albus. Journ. Morphol. 
 
 1891. 
 
 The Epiphysis of Teleosts and Amia. Loc. cit. Vol. IX. 1894. 
 JULIN, CH. De la Signification morphologique de 1'Epiphyse (Glande pineale) 
 
 des Vertebres. Bull, scientif. du Nord ; 2. Serie. X me annee. Paris, 1887. 
 KLINCKOWSTROM, A. DE. Le premier developpement de Yosil pineal, 1'epiphyse 
 
 et le nerf parietal chez Iguana tuberculata. Anat. Anz. VIII. Jahrg. 
 
 1893. (Comp. also Zoolog. Jahrb. Bd. VII. 1893.) 
 Die Zirbel und das Foramen parietale bei Callichthys (asper und littoralis. ) 
 
 Loc. cit. 
 LEYDIG, F. Das Parietalorgan der Amphibien und Reptilien. Abh. Senckenb. 
 
 Ges. Bd. XVI. 1890. Comp. also, Biol. Centralbl. Bd. VIII. 1889. 
 LUCY, W. A. The derivation of the Pineal Eye. Anat. Anz. Bd. IX. No. 5, 
 
 6, 15. 1894. (Comp. also, Journ. Morphol. Vol. VIII.) 
 ONVSIANNIKOW, PH. Ueber das dritte Auge von Petromyzon fluviatilis nebst 
 
 einigen Bemerkungen iiber dasselbe Organ dei anderen Thieren. Mem. 
 
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 PEYTOUREAU, A. La glande pineale et le troisieme ceil des Vertebres. Paris, 
 
 1887. 
 
 PRENANT, A. Sur 1'oeil parietal accessoire. Anat. Anz. Bd. IX. 1893. 
 RETZIUS, G. Ueber den Ban des sog. Parietalauges von Ammocoetes. Biol. 
 
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 RITTER, W. E. The parietal eye in some Lizards from the western United States. 
 
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 On the Presence of a Parapineal Organ in Phrynosoma coronata. Anat. Anz 
 
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 SALENSKY, W. Morph. Studien an Tunicaten. I. Ueber das Nervensystem der 
 
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 SCOTT, W. B. The parietal Eye of the Cyclostome Fishes. Journ. Morphol. 
 
 1887. 
 
 SELENKA, E. Das Stirnorgan der Wirbelthiere. Biol. Centralbl. Bd. X. 1890. 
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 Preliminary Communication on the Structure and Presence in Sphenodon and 
 
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 On the Presence and Structure of the Pineal Eye in Lacertilia. Q. Journ. 
 
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 STRAHL, H. und MARTIN, E. Die Entwicklung des Parietalauges bei Anguis 
 
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 STUDNICKA, F. CH. Sur les organes parietaux de Petromyzon Planeri. Travail 
 
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 (c) PERIPHERAL NERVOUS SYSTEM. 
 
 ADOLPHI, H. Ueber Variation der Spinalnerven und der Wirbelsaule anurer 
 
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 ANDERSON, O. A. Zur Kenntnis des sympathischen Nervensystems der urodelen 
 
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 BEARD, J. The system of Branchial Sense Organs and their associated Ganglia 
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 The Ciliary or Motor oculi ganglion and the Ganglion of the opthalmicus pro- 
 
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 The development of the Peripheral Nervous-System in Vertebrates. Part I. 
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 F F 
 
434 APPENDIX 
 
 BECK, W. Uber den Austritt des Nerv. hypoglossus [und N. cervicalis primus 
 
 aus dem Centralorgan beim Menschen u. in d. Reihe der Saugethiere unter 
 
 besond. Beriichsichtigung der dorsal. Wurzeln. Anat.Hefte. I. Abth. XVII., 
 
 Heft VI. Bd. H. II. 1895. 
 BEMMELEN, VAN. Die Halsgegend der Reptilien. Zool. Anz. X. Jahrg. 
 
 1887. 
 BERANECK, E. Rech. sur le developpement des nerfs craniaux chez les Lezards. 
 
 Recueil zoolog. suisse. T. I. 
 BOLK, L. (Seep. 415.) 
 BRANDIS, F. (See p. 429.) 
 CARLSSON, A. Beitr. z. Kennt. d. Anatomic d. Schwimmvogel. Abh. d. K. 
 
 Schwed. Vet. Acad. Bd. III. 
 Untersuchungen iiber Gliedmassenreste der Schlangen. Konigl. Schwed. 
 
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 CHEVREL, O. Sur 1'Anatomie du Systeme nerveux grand sympathique des Elas- 
 
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 OHIARUGI, G. Lo sviluppo dei Nervi vago, accessorio, ipoglosso e primi cervicali 
 
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 Osservazioni intorno alle prime fasi di sviluppo dei nervi encefalici dei 
 
 Mammiferi e in particolare sulla formazione del nervo olfattivo. Monitore 
 
 zoologico italiano, Firenze. Anno II. 1891. 
 Contrib. allo Studio dello sviluppo dei Nervi encefalici nei Mammiferi in 
 
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 COGGI, A. Ricerche su alcuni derivati dell' ectoderma nei capo dei Selaci Ricerche. 
 
 Lab. Anat. Roma e altri Lab. Biologici, Vol. V. 1895. 
 COLE, F. J. On the cranial Nerves of Chimsera monstrosa, with a discussion of 
 
 the lateral line system and of the Morphology of the Chorda tympani. Tr. 
 
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 DEYL, T. Zur vergl. Anatomic des Sehnerven. Bull. International de 1'Acad. 
 
 des Sciences de 1'Empereur Frangois Joseph I. Prag. 1895. 
 DIXON, A. L. On the Development of the Branches of the fifth cranial Nerve in 
 
 Man. Tr. Dublin Soc. Ser. II., Vol. VI. 1896. 
 
 On the course of the taste-fibres. Edinburgh Medical Journal. 1897. 
 DOHRN, A. (Seep. 426.) 
 EISLER, P. Der Plexus lumbosacralis des Menschen. Abh. Ges. Halle. Bd. 
 
 XVII. 1892. 
 EWART, J. C. and COLE, F. J. On the dorsal branches of the cranial and spinal 
 
 Nerves of Elasmobranchs. P. Roy. Soc. Edin. Vol. XX. 1895. 
 FISCHER, J. G. Die Gehirnnerven der Saurier. Hamburg, 1852. 
 
 Amphibiorum nudorum neurologiae specimen primum. Berlin, 1843. (Mull. 
 
 Arch. 1844. ) (And see p. 397. ) 
 FREUD, S. Ueber Spinalganglien und Riickenmark des Petromyzon. Sitz.-Ber. 
 
 Ak. Wien. Bd. 78, III. Abthl., 1878. 
 
 FROHSE, F. Die oberflachlichen Nerven des Kopfes. Berlin, Prag., 1895. 
 FRORIEP, A. Ueber ein Ganglion des Hypoglossus und Wirbelanlagen in der 
 
 Occipitalregion. Arch. f. Anat. und Phys. 1882. 
 Ueber Anlagen von Sinnesorganen am Facialis, Glossopharyngeus und Vagus, 
 
 iiber die genetische Stellung des Vagus zum Hypoglossus und iiber die 
 
 Herkunft der Zungenmuskulatur. Arch. f. Anat. 1885. 
 Ueber das Homologon der Chorda tympani bei niederen Wirbelthieren. Anat. 
 
 Anz. II. Jahrg. 1887. 
 FRORIEP, A., und BECK, W. Ueber das Vorkommen dorsaler Hypoglossus- 
 
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 1895. 
 FURBRINGER, M. Zur Lehre von den Umbildungen des Nervenplexus. Morph. 
 
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 Ueber die spino-occipitalen Nerven der Selachier und Holocephalen und ihre 
 
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 GADOW, H. (See p. 422.) 
 GASKELL, W. H. The structure, distribution and function of the nerves which 
 
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 1886. 
 
APPENDIX 435 
 
 GASKELL, H. W. 0*n the Relation between the Structure, Function and Distribution 
 
 of the Cranial Nerves. Prelim. Commun. P. Roy. Soc. Vol. 43. 1888. 
 On the Relation between the Structure, Function, Distribution and Origin 
 
 of the Cranial Nerves, together with a Theory of the Origin of the Nervous 
 
 System of Vertebrata. Journ. Physiol. Vol. X. No. 3. 
 <TEGENBAUR, C. Ueber die Kopfnerven von Hexanchus, etc. Jen. Zeitschr. 
 
 Bd. VI. (And see p. 410. ) 
 GORONOWITSCH, N. Unters iiber die Entwicklung de sogen. " Ganglienleisten " 
 
 im Kopfe der Vogelembryonen. Morph. Jahrb. Bd. XX. 1893. (See 
 
 p. 426.) 
 HALLER, B. Ueber den Ursprung des Nervus vagus bei den Knockenfischen. 
 
 Verh. Deutsch. Zool. Ges. Bd. V. 
 HATSCHEK. (Seep. 411.) 
 His, W. Ueber die Entwicklung des Sympathicus bei Wirbelthieren, mit 
 
 besonderer Beriicksichtigung der Herzganglien. Verh. Anat. Ges. 1892. 
 Ueber die Anfange des peripheren Nervensystems. Arch. f. Anat. u. Physiol. 
 
 1879. 
 Die Entwicklung der ersten Nervenbahnen beim menschlichen Embryo, 
 
 Uebersichtliche Darstellung. Eine kritische Studie. Arch. f. Anat. and 
 
 Physiol. 1888. 
 HOFFMANN, C. K. Zur Entwickl. -Gesch. des Selachierkopfes. Anat. Anz. 
 
 IX. Bd. 1894. 
 
 HUBER, C. CARL. The spinal ganglia of Amphibia. Anat. Anz. XII. Bd. 1896. 
 IVERSEN, M. Bemerkungen iiber die dorsalen Wurzeln des Nervus Hypoglossus. 
 
 Ber. Ges. Freiburg. Bd. II., Heft 1. 1886. 
 JHERING, H. v. Das periphere Nervensystem der Wirbelthiere, etc. Leipzig, 
 
 1878. 
 JOHNSON, A. and L. SHELDON. On the Development of the Cranial Nerves in 
 
 the Newt. P. Roy. Soc. London, 1886. 
 JULIN, CH. Le systeme nerveux grand sympathique de rAmmocoetes (Petromyzon 
 
 Planeri). Anat. Anz. II. Jahrg. 1887. 
 De la valeur morphologique du nerf lateral du Petromyzon. Bull. Ac. 
 
 Belgique. Tome XII. 1887. 
 KEY, A., und G. RETZIUS. Studien in der Anatomic des Nervensystems und des 
 
 Bindegewebes I', 11.,^. Stockholm, 1875 und 1876. 
 KRAUSE, W. Ueber die Doppelnatur des Ganglion ciliare. Morph. Jahrb. 
 
 Bd. VII. 1881. 
 
 KOLLIKER, A. Der feinere Bau und die Functionen des Sympathischen Nerven- 
 systems. Sitz.-ber. Ges. Wurzburg. Jahrg. 1894. 
 KONIGE, E. Zur Geschichte der Anatomic der Hirnnerven. Inaug. Dissert. 
 
 Freiburg i. Br. 1897. 
 KUPFFER, C. v. Ueber die Entwicklung von Petromyzon Planeri. Sitz.-ber. der 
 
 K. Bayer. Acad. der Wissensch. Miinchen, 1888. 
 Die Entwicklung der Kopfnerven der Vertebraten. Referat. Verh. Anat. 
 
 Ges. 1891. (See also pp. 426, 428.) 
 Ueber Monorhinie und Amphirhinie. Sitz.-ber. d. Math, und Physik. Cl. 
 
 d. K. Bayer. Akad. d. Wiss., 1894. Bd. XXIV. Heft 1. 
 Studien zur vergl. Entwick. Geschichte des Kopfes der Kranioten. Heft I. 
 
 Die Entwick. des Kopfes von Acipenser sturio, 1893. Heft II. Die 
 
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 F F 2 
 
436 APPENDIX 
 
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APPENDIX 437 
 
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438 APPENDIX 
 
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APPENDIX 439 
 
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440 APPENDIX 
 
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442 APPENDIX 
 
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APPENDIX 443 
 
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444 APPENDIX 
 
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446 APPENDIX 
 
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APPENDIX 447 
 
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APPENDIX 449 
 
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450 APPENDIX 
 
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APPENDIX 451 
 
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APPENDIX 453 
 
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454 APPENDIX 
 
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APPENDIX 455 
 
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456 APPENDIX 
 
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 MAMMALIA. 
 
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APPENDIX 457 
 
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458 APPENDIX 
 
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 1890. 
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 (Compare also under H.) 
 
 PISCES AND DIPNOI. 
 
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 BEMMELEN, J. F. VAN. Ueber vermuthliche rudimentare Kiemenspalten bei 
 
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 BJELETZKI. Ueber die in der Schwimmblase enthaltenen Gase. Abhdl. der 
 
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 BRIDGE, TH. W., and HADDON, A. C. (See p. 446.) 
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 HASSE, C. Beobachtungen iiber die Schwimmblase der Fische. Anat. Studien. 
 HOEVEN, VAN DER. (1) Ueber die zellige Schwimmblase des Lepidosteus. (2) 
 
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 MOBIUS, K. Balistes aculeatus, ein trommelnder Fisch. Sitz.-ber. Ak. Berlin 
 
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APPENDIX 459 
 
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460 APPENDIX 
 
 WILDER, HARRIS H. Lungenlose Salamandriden. Anat. Anz. IX. and XII. Bd. 
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APPENDIX 461 
 
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462 APPENDIX 
 
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APPENDIX 463 
 
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464 APPENDIX 
 
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472 APPENDIX 
 
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474 APPENDIX 
 
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APPENDIX 475 
 
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476 APPENDIX 
 
 GASSER, E. Die Entstehung der Cloakenoffnung bei Hiihnerembryonen. Arch. f. 
 
 Anat. u. Physiol. 1880. 
 Das obere Ende des WolfFschen Ganges. Sitz-ber. der Gesellsch. zur. Be- 
 
 forderung der gesammten Naturwiss. zu Marburg, 1878, S. 52. 
 HOFFMANN, C. K. Etude sur le developpement de 1'appareil uro-genital des 
 
 oiseaux. Verb: Ak. Amsterdam. (2. Sectie, Deel I. No. 4. 1892.) 
 KIP, M. J. VAN ERP TAALMAN. Over de Ontwikkeling van de Miiller'sche Gang 
 
 bij Zoogdieren (with abstract in German). Tijdschr. d. Ned. Dierk. Vereen. 
 
 (2). IV. 1. 1863. 
 KOWALEVSKI, R. Die Bildung der Urogenitalanlage bei Hiihnerembryonen. 
 
 Warschau, 1875. 
 KUPFFER, C. Unters. iiber die Entwicklung des Harn- und Geschlechtssystems. 
 
 Arch. f. mikr. Anat. II. 1866. 
 MULLER, JOH. Erectile mannliche Geschlechtsorgane der straussenartigen Vogel, 
 
 &c. Berlin, 1836. 
 ROMITI, W. Die Bildung des Wolff schen Ganges beim Hiihnchen. Centralbl. f. 
 
 d. medic. Wissensch. 1873. No. 31. 
 Bau und Entwicklung des Eierstockes und des Wolff schen Ganges. Arch, f . 
 
 mikr. Anat. Bd. X. 1873. 
 SEDGWICK, A. Development of the Kidney in its Relation to the Wolffian Body 
 
 in the Chick. Q. Journ. Micr. Sci. Vol. XX. 1880. 
 
 On the Development of the Structure known as the Glomerulus of the Head- 
 kidney in the Chick. Loc. tit. Vol. XX. 1880. 
 Early Development of the Wolffian Duct and Anterior Wolffian Tubules in 
 
 the Chick ; with some Remarks on the Vertebrate Excretory System. Loc. 
 
 tit. Vol. XXI, 1881. 
 SEMON, R. Die indifferente Anlage der Keimdriisen beim Hiihnchen und ihre 
 
 Differenzirung zum Hoden. Jen. Zeitschr. Bd. XXI. N. F. XIII. 1887. 
 SIEMERLING, E. Beitr. zur Embryologie der Excretionsorgane des Vogels. Inaug. - 
 
 Dissert. Marburg, 1882. 
 TICHOMIROFF, A. Androgynie bei den Vogeln. Anat. Anz. Jahrg. III. 1888. 
 
 (See also, in Russian, in Arb. des Laboratoriums des zool. Museums an 
 
 der Univ. Moskau, " Zur Frage iiber den Hermaphroditismus bei den 
 
 Vogeln.") 
 
 MAMMALIA. 
 
 AMANN, J. A., JR. Beitr. z. Morphologic der Miiller' schen Gange und iiber 
 
 accessorische Tubenostien. Arch. f. Gynakol. Bd. XLII. Heft 1. 1892. 
 ARNDT, R. Beitr. z. Anat. und Entwickl. -Geschichte des Rutheriknochens. 
 
 Inaug. -Dissert. Erlangen, 1889. 
 BEAUREGARD, H. et BOULART. Recherches sur les appareils genito-urinaires des 
 
 Balaenides. Tome XVIII. 
 BEDDARD, F. E. Remarks 011 the Ovary of Echidna. Proc. R. Physical Soc. 
 
 Edinburgh, 1885. 
 BENEDEN, E. VAN. Recherches sur la maturation de 1'oeuf, la fecondation et la 
 
 division cellulaire. Gand, Leipzig und Paris, 1883. 
 
 BISCHOFF, TH. Vergl. anat. Untersuchungen iiber die iiusseren weibl. Geschlechts- 
 organe des Menschen und der Affen. Ab. Bayer. Ak. XIII. 2. und 3. 
 BLANCHARD. Etudes sur la steatopygie et le tablier des femmes Boschimanes. 
 
 Bull. Soc. Zool. France, VIII. p. 1883. 
 BONNET, R. Ueber die ektodermale Entstehung des Wolff schen Ganges bei den 
 
 Saugethieren. Sitz.-ber. Morph. Physiol. Miinchen, 3. Jahrg. 1887. 
 
 Heft 2. 
 BRASS, A. Beitr. zur Kenntnis des weibl. Urogenitalsystems der Marsupialier. 
 
 Inaug. -Dissert. Leipzig, 1880. 
 CATTANEO, G. Sugli organi riproduttori femminili dell' Halmaturus Benetti 
 
 Gould. Milano, 1882. 
 Sulla condizione dei fondi ciechi vaginali della " Didelphys Azarae" prima e 
 
 dopo il parto. Boll. d. Mus. di zoolog. e anat. comparat. di Genova. No. 
 
 34. 1895. 
 EBNER, V. v. Unters. iiber den Bau der Samenkanalchen und die Entwicklung 
 
 der Spermatozoiden bei Saugethieren und beim Menschen. Unters. aus d. 
 
 Instit. f. Physiologic und Histologie in Graz v. A. Rollet. 2. Heft. 1871. 
 
APPENDIX 477 
 
 EGLI, TH. Beitr. z. Anatomie und Entwicklung der Geschlechts-Organe. I. 
 Zur Entw. des Urogenitalsystems beini Kaninchen. Inaug. -Dissert. 
 
 Ziirich, 1876. 
 
 FLETCHER, J. J. On the existence after parturition of a direct Communication 
 between the median vaginal cul-de-sac and the urogeriital canal in certain 
 
 Species of Kangaroos. P. Linn. Soc. N. S. W. Vol. VI. 1881. p. 796. 
 On some points in the Anatomy of the Urogenital Organs in Females of 
 
 certain species of Kangaroos, Part I., P. Linn. Soc. N. S. W. Vol. VII. 
 
 1882. p. 640, part II. Loc. cit. Vol. 1883. p. 6. 
 
 GILBERT, TH. Das Os priapi der Sauger. Morph. Jahrb. Bd. VIII. 1892. 
 HAACKE, W. Meine Entdeckung des Eierlegens der Echidna hystrix. Zool. 
 
 Anz. VII. Jahrg. No. 182. 1884. 
 HASSE, C. Die Wanderung des menschl. Eies. Zeitschr. f. Geburtshilfe und 
 
 Gynakologie. Bd. XXII. Heft 2. 
 HAYCRAFT, J. B. The development of the Kidney in the Rabbit. Internal 
 
 Monatschr. Anat. Bd. XII. Heft 6. 1895. 
 HENSEN, V. Beobacht. iiber die Befruchtung und Entwicklung des Meer- 
 
 schweinchens und Kaninchens. Arch. f. Anat. u. Physiol. 1875. 
 JARISCH, A. Ueber die Schlagadern des menschlichen Hodens. Ber. d. naturw. 
 
 Vereins zu Innsbruck. 1889. 
 KAPFF, H. Unters. iiber das Ovarium und dessen Beziehungen zum Peritoneum. 
 
 Arch. f. Anat. 1872. 
 KATZ, O. Zur Kenntnis der Bauchdecke und der mit ihr verkniipften Organe 
 
 bei den Beutelthieren. Zeitschr. f. wiss. Zool. Bd. 36. 1882. 
 KEIBEL, F. Zur Entwicklungsgeschichte der Harnblase. Anat. Anz. VI. 
 
 Jahrg. 1891. 
 Ueber die Harnblase und die Allantois des Meerschweinchens nebst einer 
 
 Bemerkung iiber die Entstehung des Nierenganges (Ureters) bei Saugern. 
 
 Anat. Anz. VIII. Jahrg. 1893. 
 
 Zur Entw. Geschichte des menschl. Urogenitalapparates. Arch. Anat. 1896. 
 (Comp. also Arch. Anat. 1888, und Verh. Anat. Ges. 1895.) 
 KLAATSCH, H. Ueber den Descensus testiculorum. Morph. Jahrb. Bd. XVL 
 
 1890. 
 KOCKS (und WASILIEFF). Ueber die Gartner'schen Canale beim Weib. Arch. f. 
 
 Gynakol. 1883. 
 KUPFFER, C. Unters. iiber die Entwicklung des Harn- und Geschlechtssystems. 
 
 Arch. f. mikr. Anat. Bd. I. 1865 u. Bd. II. 1866. 
 KOLLIKER, A. Ueber die Entwicklung d. Graafschen Follikel der Saugethiere. 
 
 Verh. Ges. Wiirzburg. Bd. VII. 1875. 
 LANDWEHR. Hermann's Physiologic (Abschnitt : Die Zeugung. Bearbeitet von 
 
 Hensen). 
 LAXGHANS, P. Ueber die accessorischen Driisen der Geschlechtsorgane. Virch. 
 
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 LATASTE. Sur le bouchon vaginal des Rongeurs. Zool. Anz. Jahrg. VI. 1883. 
 
 (Comp. also " Actes de la Societe scientif. du Chili." T. III. 1893.) 
 LISTER, J. J. and FLETCHER, J. J. On the Condition of the median portion of 
 
 the Vaginal Apparatus in the Macropodidse. P. Zool. Soc. 1881. 
 MESSING, W. Anat. Unters. iiber den Testikel der Saugethiere. Inaug. -Dissert. 
 
 Dorpat, 1877. 
 MEYER, H. Die Entwicklung der Urnieren beim Menschen. Arch. f. mikr. 
 
 Anat. Bd. XXXVI. 1890. 
 MILLER, GERRIT S., JR. On the Introitus vaginae of certain Muridae. Proc. 
 
 Boston Soc. Nat. Hist. Vol. XXVI. 
 MORAN, H. Des transformations epitheliales de la Muqueuse du vagin de 
 
 quelques Rongeurs. Journal de 1'Anat. et Physiol. Tome XXV. 
 MiiLLER, P. Das Porenfeld (Area cribrosa) oder Cribrum benedictum aut. der 
 
 Nieren des Menschen und einiger Haussaugethiere. Arch. f. Anat. u. 
 
 Phys. 1883. 
 NAGEL, W. Ueber die Entwicklung des Urogenitalsystems des Menschen. 
 
 Arch. f. mikr. Anat, Bd. XXXIV. 1889. 
 Ueber die Entwicklung des Uterus und der Vagina beim Menschen. Loc. 
 
 cit. Bd. XXXVII. 1891. 
 Ueber die Entwicklung der Urethra und des Dammes beim Menschen. 
 
 Loc. cit. Bd. XL. 1892. 
 
478 APPENDIX 
 
 PALADINO, G. Ulterior! ricerche sulla distruzione e rinnovamento continue del 
 
 parenchima ovarico nei mammiferi, etc. (Dal Laboratorio d'Istologia e 
 
 fisiolgia generale dell' University di Napoli.) Napoli, 1887. Abstract in 
 
 Anat. Anz. Jahrg. II. 1887. 
 
 PFLUGER, E. Die Eierstocke der Saugethiere und des Menschen. Leipzig, 1863. 
 PINNER, O. Ueber den Uebertritt des Eies aus dem Ovarium in die Tube beim 
 
 Saugethiere. Arch. f. Anat. und Phys. 1880. 
 POULTON, E. B. The structures in connection with the Ovarium Ovum of 
 
 Marsupialia and Monotremata. Q. Journ. Micr. Sci. Vol. XXIV. N. S. 
 
 1884. 
 RETTERER, M. ED. Note sur le developpement du penis et du squelette du 
 
 gland chez certaines rongeurs. Comptes Rendus Hebdom. des Seances 
 
 et Memoires de la Soc. du Biol. Tome IV. 1887. 
 RIESE, H. Die feinsten Nervenfasern und ihre Eudigungen im Ovarium der 
 
 Saugethiere und des Menschen. Anat. Anz. VI. Jahrg. 1891. 
 ROBINSON, A. On the Position and peritoneal Relations of the Mammalian 
 
 Ovary. Journ. Anat. and Physiol. 1887, and Stud. Owens Coll. Vol. II. 
 
 Manchester, 1890. 
 ROTH, M. Ueber einige Urnierenreste beim Menschen. Festschrift zur Feier 
 
 des SOOjahr. Bestehens der Universitat Wiirzburg. 1882. 
 SCHULIN, H. Zur Morphologic des Ovariums. Arch. f. mikr. Anat. Bd. XIX. 
 
 1881. 
 SELENKA, E. Studien iiber Entwicklungsgeschichte der Thiere. 4. Heft, II. 
 
 Halfte : Das Opossum. Wiesbaden, 1886. 
 STEINACH, E. Unters. z. vergl. Physiol. d. mannl Geschlechtsorgane. Arch f. 
 
 d. ges. Physiol. Bd. LVI. 1894. 
 
 F(ETAL MEMBRANES, PLACENTA, ETC. 
 (Comp. also pp. 468-478). 
 
 BEARD, JOHN. The yolk-sac, yolk, and merocytes in Scyllium and Lepidosteus. 
 
 Anat. Anz. Bd. XII. 1896. 
 BENEDEN, E. VAN and JULIN, CH. Recherches sur la formation des annexes 
 
 fcetales chez les Mammiferes. Arch. Biol. Tome V. 1884. 
 BONNET, R. Die Uterinmilch und ihre Bedeutung fiir die Frucht. Stuttgart, 
 
 1882. 
 BUMM E. Ueber die Entwicklung des miitterlichen Kreislaufes in der mensch- 
 
 lichen Placenta. Arch. f. Gynakologie. Bd. XLIII. Heft 2. 1892. 
 Ueber die Entwiklung des miitterl. Blutkreislaufes in der menschl. Placenta. 
 
 Arch. f. Gynakol. Bd. XLIII. H. 2. 
 
 CALDWELL, W. H. On the Arrangement of the Embryonic Membranes in Mar- 
 supial animals. Q. Journ. Micr. Sci. Vol. XXIV. 1884. 
 The Embryology of Monotremata and Marsupalia. Phil. Trans. Vol. 178. 
 
 1887. 
 
 DUVAL, M. Le Placenta des Rongeurs. Paris, 1889-93. 
 FLEISCHMANN, A. Der einheitliche Plan der Placeiitarbildung bei Nagethieren. 
 
 Sitz-ber. Ak. Berlin. XXVI. 1892. Comp. also " Embryol. Untersuch- 
 
 ungen." H. 1-3. Wiesbaden, 1889-93. 
 Embryol. Untersuchungen III. Heft. Die Morphol. d. Placenta bei Nagern 
 
 und Raubthieren. Wiesbaden, 1893. 
 FROMMEL, R. Ueber die Entwicklung der Placenta von Myotus murinus. 
 
 Wiesbaden, 1888. 
 GIACOMINI, E. Nuovo Contribute alia migliore conoscenza degli annessi fetali nei 
 
 Rettili. Recezione del sacco vitellino e dell'allantoide nella cavita 
 
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 Agosto, 1893. 
 GODET, R. Rech. sur la structure intime du Placenta du Lapin. Inaug-Diss. 
 
 Bern, 1877. 
 HEINRICIUS, G. Ueber die Entwicklung und Structur der Placenta beim Hunde. 
 
 Arch. f. mikr. Anat. Bd. XXXIII. 1889. 
 
 HILL, J. P. On the placenta of Perameles obesula. P. Linn. Soc. , N.S. W. 1895. 
 HILL, J. P., and MARTIN, C. J. On a Platypus embryo from the intra-uterine 
 
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APPENDIX 479 
 
 HIROTA, S. On the sero-amniotic connection in the chick. Journ. Coll. Sci., 
 
 Japan, Vol. VI. 1894. 
 HOFMEIER, M. Beitr. z. Anat. u. Entwick. d. menschl. Placenta. Zeitschr. f. 
 
 Geburtshilfe u. Gynakologie. Bd. XXX., H. 3. 
 ECHT, A. A. W. Spolia Nei 
 
 HUBRECHT, A. A. W. Spolia Nemoris (includes Lemurs and Edentates). Q. 
 
 Journ. Micr. Soc. Vol. XXXVI. 1894. 
 The Placentation of the Shrew (Sorex vulgaris). Loc. cit. Vol. XXXV. 
 
 1893. 
 Die Phylogenese des Amnions und die Bedeutung des Trophoblastes. Verh. 
 
 Ak. Amsterdam, 1895. 
 KEIBEL, FR. Zur Entwicklungsgeschichte der menschlichen Placenta. Anat. 
 
 Anz. 1889. No. 17. 
 LANGHANS, TH. Unters. iib. die menschliche Placenta. Arch. f. Anat. u. Phys. 
 
 1877. 
 MIXOT, CHARLES SEDGWICK. Uterus and Embryo. I. Rabbit. II. Man. Journ. 
 
 Morphol. Vol. II. 1889. 
 
 Die Placenta des Kaninchens. Biol. Centralbl. Bd. X. 1890. 
 A theory of the structure of the placenta. Anat. Anz. VI. Jahrg. No. 5. 
 1891. 
 
 Uterus and Embryo. Journ. Morphol. 1889. 
 MITSUKURI, K. On the foetal membranes of the Chelonia. Journ. Coll. Sci., 
 
 Japan, Vol. IV. 1890. 
 OSBORX, H. F. The Fcetal Membranes of the Marsupials. Journ. Morphol. Vol. 
 
 I. 1887. (See also Q. Journ. Micr. Sci., 1883.) 
 PARKER, T. J. Note on the foetal membranes of Mustelus aiitarcticus. Tr. 
 
 N. Z. Inst. Vol. XXII. 1889. 
 
 POPOFF, D. Die Dottersask-Gefasse des Huhnes. Wiesbaden, 1894. 
 ROBINSON, A. The Nutritive Importance of the Yolk-Sac. Journ. Anat. and 
 
 Physiol. Vol. XXVI. 1892. 
 RYDER, J. A. The origin of the Amnion. Amer. Nat. Vol. XX. 1886. (And 
 
 compare also Vol. XXI. 1887.) 
 SELENKA, E. Zur Entstehung der Placenta des Menschen. Biol. Centralbl. 
 
 Bd. X. No. 24. 1891. 
 
 SEMON, R. Entstehung u. Bedeutung der embryonalen Hiillen u. Anhangs- 
 organe d. Wirbelthiere. Cornpte-rendu des Seances du troisieme Congres 
 internat. de Zoologie. Leyde, 1895. 
 Zool. Forschungreisen in Australien u. dem Malayishen Archipel. Bd. II. 
 
 Jena, 1894. (See also p. 470.) 
 STRAHL, H. See numerous papers 011 the Placenta in Arch. f. Anat. und Physiol. 
 
 and Marburger Sitzungsberichten fom Jahr, 1888, onwards. 
 Der Uterus post partum I. Anat. Hefte. I. Abth. Bd. III. H. 3. 1894. 
 Der puerperale Uterus der Hundin. Loc. cit. I. Abth. Bd. V. H. 3. 
 
 1895. 
 
 TAFANI. Sulle condizioni uteroplacentari della vita fetale. Firenze, 1886. 
 TURNER, W. Observations on the structure of the human placenta. Journ. 
 
 Anat. and Physiol. Vol. VII. 1873. 
 Some general observations on the placenta with special reference on the 
 
 theory of evolution. Loc. cit. Vol. XI. 1877. 
 Lectures on the anatomy of the placenta. Edinburgh, 1876. 
 WALDEYER, W. Bemerkungen iiber den Bau der Menschen- und Affenplacenta 
 
 Arch. f. mikr. Anat. Bd. XXXV. 1890. 
 Ueber den Placentarkreislauf des Menschen. Sitz.-ber. Ak. Berlin. VI. 
 
 1887. 
 WIEDERSHEIM, R. Beitr. zur Entw.-Gesch. von Salamandra atra. Arch. f. 
 
 mikr. Anat. Bd. XXXVI. 1890. 
 WILSON, J. T. Description (with figures) of a young specimen of Ornithorhyn- 
 
 chus anatinus. P. Linn. Soc., N.S.W. Vol. IX. 
 
 WOOD-MASON, J. and ALCOCK, A. On the uterine villiform papillae of Ptero- 
 platea micrura and their relation to the embryo. P. Roy. Soc. Vol. XLIX. 
 1891. 
 
 ZIEGLER, H. E. Die Entstehung des Periblastes bei der Knochenfischen. Anat. 
 Anz. Bd. XII. 1896. 
 
480 APPENDIX 
 
 SUPRARENAL BODIES. 
 
 ALEXANDER, C. Untersuchungen iiber die Nebenniere und ihre Bezielmngen zum 
 
 Nervensystem. Ziegler's Beitr. d. path. Anat. u. allg. Path. Bd. XI. 
 
 Heft 1. 1891 V S. 145-197. 
 BRAUN, M. Ueber den Ban und die Entwicklung der Nebennieren bei Reptilien. 
 
 Arb. Inst. Wiirzburg. Bd. V. 
 BRUNN, A. v. Ein Beitrag zur Kenntnis des feineren Baues und der Entwick- 
 
 lungsgeschichte der Nebennieren. Arch, f. mikr. Anat. Bd. VIII. 1872. 
 COLLINGE, W. E. On the so-called suprarenal bodies in Cyclostoma. Anat. Anz. 
 
 Bd. XII. 1896. 
 
 The suprarenal bodies of Fishes. Nat. Sci. Vol. X. 1897. 
 DOSTOIEWSKY, A. Ein Beitrag zur mikroskop. Anatomic der Nebennieren bei 
 
 Saugethieren. Arch. f. mikr. Anat. Bd. XXVII. 1886. 
 GOTTSCHAU, M. Ueber Nebennieren der Saugethiere, speciell iiber die des 
 
 Menschen. Sitz.-ber. Ges. Wiirzburg, 1882. 
 Ueber die Nebennieren der Saugethiere. Biol. Centralbl. Bd. III. 1883. 
 
 No. 18. 
 Structur und embryonale Entwicklung der Nebennieren bei Saugethieren. 
 
 Arch. f. Anat. u. Physiol. 1883. 
 JANOSIK. Bemerkungen iiber die Entwicklung der Nebenniere. Arch. f. mikr. 
 
 Anat. Bd. XXII. 1883. 
 MASAMARO INABA. Notes on the Development of the Suprarenal Bodies in the 
 
 Mouse. Journ. Coll. Sci. Japan. Vol. IV. 1891. 
 MIHALCOVICS, v. (See p. 469.) 
 MITSUKURI. On the Development of the Suprarenal Bodies in Mammalia. Q. 
 
 Journ. Micr. Sci. London. New Series. 1882. 
 PETIT, A. Recherches sur les capsules surrenales. Theses presentees a la 
 
 faculte des sciences de Paris, etc. Paris, 1896. (See also Bull. Soc. Zool. 
 
 France. XX.) 
 PFAUNDLER, M. Zur Anatomic der Nebenniere. Sitz.-ber. Ak. Wien. Bd. 
 
 CI. Abth. III. 1892. 
 
 RAUBER. Zur feineren Structur der Nebennieren. Inaug.-Diss. Berlin, 1881. 
 STILLING, H. Zur Anatomic der Nebennieren. Virchow's Arch. Bd. C. IX. 
 
 1887. 
 VINCENT, SWALE. The suprarenal capsules in the lower Vertebrates. Proc. 
 
 Birmingham. Nat. Hist, and Philosoph. Soc. Vol. X. Part 1. 1896. 
 On the morphology and physiology of the suprarenal capsules in Fishes. Anat . 
 
 Anz. Bd. XIII. 1897. 
 Contribution to the Comparative Anatomy and Histology of the suprarenal 
 
 capsules. The suprarenal bodies in Fishes, and their relation to the 
 
 so-called head kidney. Tr. Zool. Soc. Vol. XIV. Part 3. 1897. 
 WELDON, W. F. R. On the Head Kidney of Bdellostoma with a suggestion as to 
 
 the origin of the Suprarenal Bodies. Stud. Morph. Cam. Vol. II. Part 1. 
 
 1884. 
 
 On the suprarenal Bodies of Vertebrata. Q. Journ. Micr. Sci. 1885. 
 ZANDER, R. Ueber functionelle und genetische Beziehungen der Nebennieren 
 
 zu andern Organen, speciell zum Grosshirn. Beitr. z. pathol. Anat. und 
 
 z. allgem. Pathol., herausgegeb. von Prof. E. Ziegler. Bd. VII. 1890. 
 
INDEX 
 
 AMPHIOXUS : segmentation of ovum, 5 ; 
 development of body-cavity, 8 ; inte- 
 gument, 16 ; notochord, 36 ; central 
 nervous system, 157 ; nerves, 178, 179, 
 189 ; sense-organs, 190, 197, 211 ; 
 lateral or metapleural folds, 104 ; 
 muscles, 137 ; mouth, 239 ; alimentary 
 canal, 257, 267 ; liver, 269 ; gills, 275 ; 
 blood-vessels and corpuscles, 299 1 
 urinary organs, 348 ; generative organs, 
 359 (Compare Fig. 219) 
 
 CYCLOSTOMES : segmentation of ovum, 5; 
 integument, 16 ; vertebral column, 36 ; 
 skull, 72 ; median fins, 103 ; muscles, 
 137, 142 ; brain, 157 ; spinal nerves, 
 179, 189 ; sense-organs of the integu- 
 ment, 191, 193 ; olfactory organ, 197 ; 
 eye, 211 ; auditory organ, 224 ; mouth, 
 239 ; horny teeth, 241 ; tongue, 252 ; 
 thyroid, 255 ; alimentary canal, 257, 
 259, 267 ; liver, 271 ; pancreas, 272 ; 
 gills, 275 ; genital pores, 298 ; blood- 
 corpuscles, 300 ; heart, 305 ; veins, 
 322, 326 ; urinary organs, 349 ; genera- 
 tive organs, 359 
 
 FISHES : integument, 16 ; exoskeleton, 
 30 ; vertebral column, 37 ; ribs, 54 ; 
 skull, 74-81 ; unpaired fins, 103 ; 
 paired fins, 104 ; pectoral arch, 106 ; 
 pelvic arch, 109 ; free limbs, 122 ; 
 parietal muscles, 137 ; visceral muscles, 
 142 ; muscles of the appendages, 142 ; 
 electric organs, 146 ; spinal cord, 151 ; 
 brain, 159-165 ; spinal nerves, 179 ; 
 cerebral nerves, 180 ; sympathetic, 188; 
 sense-organs of the integument, 190- 
 193; olfactory organ, 198; eye, 211 ; 
 retina, 214 ; eye-muscles and eyelids, 
 216, 217 ; auditory organ, 224 ; rela- 
 tions of auditory organ with air- 
 bladder, 226 ; teeth, 241 ; tongue, 252 ; 
 thyroid, 255 ; thymus, 256 ; alimentary 
 canal, 257, 267 ; liver, 269 ; pancreas, 
 
 272 ; gills, 273, 276 ; air-bladder, 273, 
 280 ; abdominal and genital pores, 298 ; 
 heart and vessels, 300, 305 ; arteries, 
 319 ; veins, 302 ; retia mirabilia, 333 ; 
 lymphatic system, 333 ; spleen, 335 ; 
 nutrition of embryo, 336 ; urinary 
 organs, 350 ; generative organs, 360 ; 
 claspers, 377 ; suprarenal bodies, 385 
 
 DIPNOANS : integument, 16 ; exoskele- 
 ton, 31 ; vertebral column, 37 ; ribs, 
 54 ; skull, 81 ; unpaired fins, 103 ; 
 pectoral arch, 107; pelvic arch, 111 ; 
 free limbs, 122 ; parietal muscles, 137 ; 
 muscles of the appendages, 142 ; brain; 
 165 ; cerebral nerves, 180 ; sense- 
 organs of the integument, 190-193 ; 
 olfactory organ, 199; eye, 211 ; audi- 
 tory organ, 224 ; teeth, 243 ; tongue, 
 252.; thyroid, 255 ; thymus, 257 ; ali- 
 mentary canal, 257, 267 ; pancreas, 
 272 ; gills, 278 ; lungs, 283, 288 ; ab- 
 dominal pore, 278 ; heart, 307 ; blood- 
 vessels, 319, 326 ; spleen, 335 ; urinary 
 organs, 352 ; generative organs, 361 
 
 AMPHIBIANS : segmentation of ovum, 
 5 ; integument, 18 ; exoskeleton, 20, 33; 
 vertebral column, 42 ; ribs, 55 ; sternum, 
 58 ; episternum, 62 ; skull, 82, 88 ; 
 median fins, 183 ; pectoral arch, 107 ; 
 pelvic arch, 111 ; free limbs, 127 ; 
 parietal muscles, 137 ; visceral 
 muscles, 143 ; muscles of the appen- 
 dages, 142 ; spinal cord, 152 ; brain, 
 166 ; spinal nerves, 179 ; cerebral 
 nerves, 180 ; sympathetic, 189 ; sense- 
 organs of the integument, 190, 193 ; 
 tactile cells, 175 ; olfactory organ, 200 ; 
 Jacobson's organ, 205 ; eye, 212 ; 
 retina, 214 ; eye-muscles and eyelids, 
 216, 217 ; glands of the eye, 218 ; 
 auditory organ, 226 ; teeth, 243 ; glands 
 of the mouth, 251 ; tongue, 253 ; thy- 
 roid, 255 ; thymus, 257 ; alimentary 
 
 I I 
 
482 
 
 INDEX 
 
 canal, 257, 267 ; liver, 269 ; pancreas, 
 272 ; gills, 273, 279 ; air-tubes and 
 larynx, 283 ; lungs, 288 ; blood-cor- 
 puscles, 300 ; heart, 309 ; arteries, 319 ; 
 veins, 328 ; lymphatic system, 333 ; 
 spleen, 335 ; nutrition of embryo, 336 ; 
 urinary organs, 352 ; generative organs, 
 365 ; copulatory organs, 379 ; adrenals, 
 385 
 
 REPTILES : segmentation of ovum, 5 ; 
 integument, 20 ; exoskeleton, 33 ; 
 vertebral column, 45 ; ribs, 56 ; ster- 
 num, 60 ; episternum, 63 ; skull, 88 ; 
 median fins, 103 ; pectoral arch, 108 ; 
 pelvic arch, 113 ; free limbs, 127 ; 
 parietal muscles, 138 ; visceral muscles, 
 144 ; muscles of the appendages, 142 ; 
 "diaphragm," 141; spinal cord, 152; 
 brain, 167 ; spinal nerves, 179 ; cere- 
 bral nerves, 180 ; sympathetic, 189 ; 
 end-buds, 193 ; tactile cells, 195 ; Pa- 
 cinian corpuscles, 195 ; olfactory organ, 
 201 ; Jacobson's organ, 207 ; retina, 
 214 ; eye-muscles and eyelids, 216, 217 ; 
 glands of eye, 218 ; auditory organ, 
 227 ; teeth, 243 ; glands of mouth, 251 ; 
 tongue, 253 ; thyroid, 255 ; thymus, 
 257 ; alimentary canal, 262, 267 ; liver, 
 269 ; pancreas, 272 ; air-tubes and 
 larynx, 284 ; lungs, 290 ; abdominal 
 pores, 298 ; heart, 313 ; arteries, 319 ; 
 veins, 328 ; lymphatic system, 333 ; 
 spleen, 335 ; urinary organs, 356 ; 
 generative organs, 368 ; copulatory 
 organs, 379 ; suprarenals, 370, 385 
 
 BIRDS : segmentation of ovum, 5 ; inte- 
 gument, 20 ; vertebral column, 47 ; 
 ribs, 56 ; sternum, 60 ; episternum, 63 ; 
 skull, 93 ; pectoral arch, 109 ; pelvic 
 arch, 119; limbs, 129; parietal 
 muscles, 140 ; spinal cord, 152 ; brain, 
 172 ; cerebral nerves, 180 ; sympathetic, 
 189 ; tactile cells, 195 ; Pacinian cor- 
 puscles, 195 ; olfactory organ, 202 ; 
 eye, 213 ; retina, 214 ; eye-muscles and 
 eyelids, 216, 217 ; glands of the eye, 
 218 ; auditory organ, 227 ; teeth, 245 ; 
 glands of the mouth, 252 ; tongue, 253 ; 
 thyroid, 256 ; thymus, 257 ; alimen- 
 tary canal, 262, 267 ; liver, 269 ; pan- 
 creas, 272 ; air-tubes and larynx, 
 285 ; lungs and air-sacs, 291 ; circu- 
 lation in embryo, 364 ; heart, 315 ; 
 arteries, 319 ; veins, 328 ; lymphatic 
 system, 334 ; urinary organa, 356 ; 
 generative organs, 368 ; copulatory 
 organs, 380 ; suprarenals, 385 
 
 MAMMALS : segmentation of ovum, 5 ; 
 integument, 23 ; mammary glands, 27 ; 
 exoskeleton, 34 ; vertebral column, 49 ; 
 ribs, 57 ; sternum, 60 ; episternum, 63 ; 
 
 skull, 96 ; median fins, 103 ; pectoral 
 arch, 109 ; pelvic arch, 120 ; limbs, 
 130 ; parietal muscles, 140 ; visceral 
 muscles, 144 ; muscles of appendages, 
 142 ; diaphragm, 141 ; spinal cord, 152 ; 
 brain, 172 ; spinal nerves, 179 ; cere- 
 bral nerves, 180 ; sympathetic, 189 ; 
 end-buds, 193 ; tactile cells, 195 ; Pa- 
 cinian corpuscles, 195 ; olfactory organ, 
 203 ; Jacobson's organ, 207 ; eye, 214 ; 
 retina, 214 ; eye-muscles and eyelids, 
 216, 217 ; glands of eye, 218 ; auditory 
 organ, 229 ; histology of cochlea, 232 ; 
 lips, 239 ; teeth, 245 ; glands of mouth, 
 252 ; tongue, 255 ; thyroid, 256 ; thy- 
 mus, 257 ; alimentary canal, 263, 267 ; 
 liver, 269 ; pancreas, 272 ; air-tubes 
 and larynx, 286 ; lungs, 296 ; blood- 
 corpuscles, 300 ; heart, 315 ; arteries, 
 319 ; veins, 328 ; lymphatic system, 
 334 ; spleen, 335 ; tonsils, 335 ; urinary 
 organs, 358 ; generative organs, 370 ; 
 copulatory organs, 382 ; suprarenals, 
 385 
 
 A. 
 
 Abdominal pores, 298 
 Acetabular bone, 120 
 Acetabulum, 113-120 
 Achromatin, 3 
 Acrania, 13 
 
 Acrodont dentition, 243 
 Acromion, 109 
 Adrenals (see Suprarenals) 
 Air-bladder, 273, 280 
 Air-sacs of birds, 291 
 Air-tubes, 283 
 Alimentary canal, 235-269 
 
 appendages of, 269 
 
 mucous membrane of, 267 
 Allantois, 9, 259, 337 
 Amnion, 9, 337 
 Amniota, 9 
 
 Amphicoelous vertebrae, 40 
 Anamnia, 9 
 Antibrachium, 126 
 Antlers, 100 
 Anus, 235 
 Aortic arches, 303 
 Aponeurosis, pulmonary, 292 
 Appendages, 12 
 Appendices auricula}, 305 
 Apteria, 21 
 Arachnoid, 151 
 Archenteron, 5 
 Arches, neural and haemal, 36 
 Archipterygium, 106, 124 
 Arteries, 299322 
 Artiodactyle foot, 134 
 Arytenoid cartilages, 283 
 Astragalus, 127132 
 Atlas and axis of Reptiles, 46 ; of Birds, 
 48 ; of Mammals, 49, 50 
 
INDEX 
 
 483 
 
 Atrial chamber of Amphioxus, 275 
 Auditory capsules, 68 
 Auditory organ, 220 234 : development 
 of, 220 ; relations with air-bladder, 226 
 Auditory ossicles, 100, 231 
 Autostylic skulls, 75 
 
 B. 
 
 Baleen, 26 
 
 Basal processes of vertebral column, 38, 
 54 
 
 Basilar plate, 67 
 
 Basipterygium, 103, 104, 110, 122125 
 
 Bidder's organ, 366 
 
 Bile-duct, 272 
 
 Biserial fin, 106, 123, 124 
 
 Blastoderm, 4 
 
 Blastopore, 5 
 
 Blastosphere, 4 
 
 Blastula, 4 
 
 Blood corpuscles, 299, 300 
 
 Blood vessels, 299 
 
 Bodies of vertebrae (see Centra) 
 
 Body-axis, 12 
 
 Body-cavity, 8 
 
 Bones, cartilage-, membrane-, and invest- 
 ing-, 70 ; dermal, 18, 20, 26 
 
 Bones of skull (see Skull) 
 
 Brachium, 126 
 
 Brain : development, 149, 153 ; mem- 
 branes of, 151 ; general structure, 153 ; 
 convolutions, 154, 172 ; epiphysis, 154, 
 155 ; hypophysis, 154, 155 ; optic 
 vesicles, 154 ; pallium, 153 ; saccus 
 vasculosus, 154, 159, 160 ; ventricles, 
 156 
 
 Brain of Cyclostomi, 157 ; of Elasmo- 
 bratichii and Holocephali, 159 ; of 
 Ganoidei, 162 ; of Teleostei, 162 ; of 
 Dipnoi, 165 ; of Amphibia, 166 ; of 
 Reptilia, 167 ; of Aves, 172 ; of Mam- 
 malia, 172 
 
 Brain-case, 67 
 
 Branchiae (see Gills) 
 
 Branchial arches, 69, 7580. 85, 88, 93. 
 94, 102 
 
 Branchial basket of Cyclostomes, 73, 74 
 
 Branchial clefts, 69, 72, 236, 273 
 
 Branchiostegal membrane and rays, 79, 
 82 
 
 Bronchi, 281, 285, 296 
 
 Bursa Fabricii, 263 
 
 C. 
 
 Caecum, 236, 262, 266 
 Calcaneum, 126133 
 Campanula Halleri, 212 
 Cannon-bone,' 134 
 Capillaries, 300 
 
 Carapace, 34 
 
 Carpalia, 126133 
 
 Carpometacarpus, 130 
 
 Carpus, 126133 
 
 Cartilage-bones, 71 
 
 Cauda equina, 152 
 
 Cement of teeth, 240 
 
 Centra of vertebrae, 36 
 
 Central nervous system, 11, 149 
 
 Centrosome 3, 348 
 
 Cerebral flexure, 156 
 
 Cerebral nerves, 180 
 
 Cerebral vesicles, 153 
 
 Cerebro-spinal cavity, 11 
 
 Cheiropterygium, 125 
 
 Chevron bones, 46, 52 
 
 Chiasma, optic, 208 
 
 Choanae, 82 
 
 Chondrocranium, 69 
 
 Chorda dorsalis (see Notochord) 
 
 Chorion, 338 
 
 Choroid, 209 
 
 Choroid fissure, 209 
 
 Choroid "gland," 212 
 
 Choroid plexus, 158 
 
 Chromatin, 3 
 
 Cilia, 16 
 
 Ciliary folds, 209 
 
 Ciliary muscles, 210 
 
 Circulation (foetal), 300 
 
 Claspers, 31, 377 
 
 Classification of Vertebrates, 13 
 
 Clavicle, 107109 
 
 Claws, 16, 19, 20, 21, 26, 130 
 
 Clitoris, 373, 380, 384 
 
 Cloaca, 236, 259, 262 
 
 Coccyx, 52 
 
 Cochlea, 221232 ; histology of, 233 
 
 Coelome, 8 
 
 Colon, 236 
 
 Colostrums, 29 
 
 Columella auris, 84 
 
 Commissures of brain, 154 
 
 Conjunctiva, 210 
 
 Constriction of notochord, 39, 40, 42, 45, 
 
 47, 49. 
 
 Copulatory organs, 377 
 Coracoid, 107109 
 Corium, 16 
 Cornea, 210 
 
 Corpora adiposa, 368, 370 
 Corpora cavernosa and corpus spongio- 
 
 sum, 382, 384 
 Corpus callosum, 173 
 Corpus luteum, 347 
 Corpuscles of blood, 268 
 Craniata, 13 
 Cranium, 66, 67 
 Cribriform plate, 99 
 Cricoid cartilage, 283 
 Crop, 262 
 Crus, 126 
 Cuboid, 132 
 Cutis, 16 
 
484 
 
 INDEX 
 
 I). 
 
 Decidua, 340 
 
 Dental formula?, 250 
 
 Denticles, dermal, 30, 71 
 
 Dentine, 240 
 
 Dentition, milk, 245 
 
 Dermal skeleton, 3034 
 
 Dermis, 16 
 
 Deuteroplasm, 3, 4 
 
 Development : general, 3 12; feathers, 
 21 ; hairs, 23 ; teats, 28 ; dermal skele- 
 ton, 30 ; vertebral column, 34 ; tail of 
 Fishes, 41 ; ribs, 52 ; sternum, 58 ; 
 skull, 6472 ; horns, 99 ; limbs, 
 102106 ; muscles, 135, 142 ; electric 
 organs, 147 ; central nervous system, 
 149; brain, 153; nerves, 177, 180; 
 sympathetic, 188 ; sensory organs, 
 189 ; olfactory organ, 196 ; eye, 207 ; 
 glands of eye, 218 ; auditory organ, 
 220 ; alimentary canal, 235 ; teeth, 
 239 ; thyroid, 255 ; thymus, 256 ; ali- 
 mentary glands, 267272 ; gills, 272 ; 
 air-bladder, 273 ; lungs, 281 ; air-sacs, 
 295 ; heart, 300 ; placenta, 337 ; 
 urinogenital organs, 341 ; suprarenals, 
 385 
 
 Diaphragm, 141 
 
 Digestion, intraeellular, and extracellu- 
 lar, 267 
 
 Digits, 123134. 
 
 Diphycercal tail, 41 
 
 Diphyodont, 241 
 
 Discoid segmentation, 5 
 
 Duct : hepatic, 272 ; naso-lachrymal, 
 201 ; naso-palatine of Myxinoids, 198 ; 
 pancreatic, 272 ; pneumatic, 280 ; sali- 
 vary, 251, 252 ; urinogenital, 346. 
 
 Ductus Botalli, 312. 
 Cuvieri, 322 
 ejaculatorius, 377 
 endolymphaticus, 221 
 perilymphaticus, 232 
 venosus, 333 
 
 Duodenum, 236 
 
 Dura mater, 151 
 
 E. 
 
 Ear, 220234 
 
 Echeneis, suctorial disk, 103 
 Ectoderm, 4 
 Egg-cell (see Ovum) 
 Electric lobes of brain, 161 
 Electric organs, 146, 150 
 Embryonic area, 8 
 Enamel organs, 240 
 End-buds, 193 
 Endoderm, 4 
 Endolymph, 222 
 Ensiform process, 61 
 Enterocceles, 8 
 
 Epiblast, 4 
 
 Epicoracoid, 109 
 
 Epidermis, 16 
 
 Epididymis, 346, 350 
 
 Epiglottis, 286 
 
 Epiphyses of vertebrse, 49 
 
 Epiphysis cerebri, 155 
 
 Epipubis, 111, 114, 117, 118, 121 
 
 Episternum, 6264 
 
 Eustachian aperture and tube, 87 91, 94 
 
 Exoskeleton, 30, 34 
 
 Extra-branchials, 73, 75 
 
 Eye, 207216: glands in connection 
 
 with, 217 ; muscles of, 216 
 Eyelids, 217 
 Eyes, rudimentary, 211 213 
 
 F. 
 
 Fallopian tube, 373 
 
 Fascia, 136 
 
 Fat-bodies, 368, 370 
 
 Feathers, 21 
 
 Femur, 126131 
 
 Fenestra : rotunda, 91, 227, 232 ; ovalis. 
 
 84, 226, 232 
 
 Fertilisation of ovum, 3 
 Fibula, 126, 131 
 Eibulare, 126, 133 
 Filum terminate, 152 
 Fin-rays, 103, 105, 122, 125 
 Fins (see Limbs) 
 Food-yolk, 3, 5 
 Foramen ovale (of heart), 317 
 Foramen Panizzre, 314 
 Furcula, 109 
 
 Gall-bladder, 272 
 
 Gartner's duct, 375 
 
 Gastrula, 5 
 
 Generative cells, development of, 347 
 
 Generative ducts, 346 
 
 Generative organs, 359 385 
 
 Genital pores, 298 
 
 Germinal epithelium, 347 
 
 Germinal layers, 4 
 
 Germinal spot, 3 
 
 Germinal vesicle, 3 
 
 Gill-arches and clefts (see Branchial 
 
 arches and Clefts) 
 Gills, 236, 273 
 
 Gills, external, 273, 278, 279 
 Gills, spiracular, 278 
 Gizzard, 262 
 Glands : Bowman's, 204 ; of Bartholini, 
 
 385 ; of claspers, 18, 378 ; Cowper's, 
 
 385 ; digestive, 267 ; femoral, 27 ; 
 
 gastric, 257, 267 ; Harderian, 217 ; 
 
 inguinal, 27; integumentary, 1729; 
 
 intermaxillary or internasal, 251 ; 
 
INDEX 
 
 485 
 
 labial, 251 ; lachrymal, 217 ; of Lieber- 
 kiihn, 268 ; lingual, 251 ; mammary, 
 27 ; Meibomian, 219 ; Moll's, 219 ; 
 nasal (external) of Birds, 203 ; of olfac- 
 tory mucous membrane, 201, 202 ; ova- 
 ducal, 363 ; palatine, 251 ; parotid, 
 252 ; pharyngeal, 251 ; poison, 18, 251 : 
 preputial, 27, 385 ; prostate, 377 ; 
 rectal, 261 ; sebaceous, 27 ; Stenson's, 
 204 ; sublingual, 252 ; sweat, 27 ; uni- 
 cellular, 217 ; uropygial, 21. 
 
 Glomerulus, 345 
 
 Glomus, 341 
 
 Glottis, 281 
 
 Glyptodon, exoskeleton of, 34 
 
 Gnathostomata, 73 
 
 Goblet-cells, 17 
 
 Gonads, 347 
 
 Gut, postanal, 322 
 
 Gyri, 154 
 
 H. 
 
 Hairs, 16, 24 
 
 Head, 12 
 
 Heart, 299319 
 
 Hemibranch, 276 
 
 Heredity, 1 
 
 Hermaphrodite structures, 365, 366. 370 
 
 Heterocercal tail, 41 
 
 Heterodont dentition, 241 
 
 Hibernating gland, 335 
 
 Holoblastic segmentation, 5 
 
 Holobranch, 276 
 
 Homocercal tail, 41 
 
 Homodont dentition, 241 
 
 Horns, 99 
 
 Humerus, 126133 
 
 Humour, vitreous, 209 
 
 aqueous, 210 
 Hyaloplasm, 3 
 Hymen, 375 
 Hyoid arch, 69, 70, 75, 76, 80, 82, 85, 
 
 88, 93, 94, 102 
 Hyomandibular, 70, 75, 76 
 Hyostylic skulls, 75 
 Hypoblast, 4 
 Hypoischium, 117, 118 
 Hypophysis cerebri, 155 
 Hypural bones, 41 
 
 Ichthyopsida, 13 
 
 Ichthyopterygium, 125 
 
 Ileum, 236 
 
 Ilium, 114120 
 
 Impregnation, 3 
 
 Incus, 100, 231 
 
 Inguinal canal, 375 
 
 Integument, 16 29; sense, organs, of, 190 
 
 Intercalary pieces of vertebrae, 38 
 
 Intercentra, 41, 44, 45, 47, 52 
 Interclavicle, 63 
 Intermedium, 126133 
 Intermuscular bones, 55 
 Interspinous bones, 103 
 Intertrabecula, 67, 74 
 Intervertebral discs, 46, 48, 49 
 Intestine, small and large, 236, 257. 262, 
 
 263 
 
 Iris, 210 
 Ischium, 114, 120 
 
 J, 
 
 Jacobson, anastomosis of, 186 
 Jacobson, organ of, 205 
 Jejunum, 236 
 
 K. 
 
 Karyokinesis, 3 
 Kidney, 340359 
 
 Labial cartilage, 73, 75 
 
 Labyrinth : membranous, 221 : bonv, 
 222 
 
 Lachrymal glands. 217 
 
 Lacteals, 334 
 
 Lagena, 221 
 
 Lamina cribrosa, 74, 85 
 
 Lanugo, 26 
 
 Laryngeal pouches, 288 
 
 Laryngo-tracheal chamber, 283 
 
 Larynx, 281, 283 
 
 Lateral fin-folds, 103 
 
 Lateral line, sensory organs of, 191 
 
 Lens, crystalline, 209 
 
 Leucocytes, 334 
 
 Ligaments, intervertebral, 36 
 
 Limbs : unpaired, 102; paired, 103134 
 
 Lips, 239 
 
 Liver, 269 
 
 Lungs, 273, 281, 288 
 
 Lymph, 299, 334 
 
 Lymph -hearts, 334 
 sinuses, 334 
 vessels, 299, 333 
 
 Lymphatic glands. 267, 335 
 system, 333 
 
 Lymphoid substance in relation with 
 urinogenital organs : of Teleostei, 
 Ganoidei, and Dipnoi, 352, 363 : of 
 Amphibia, 368 ; of Reptilia, 370 
 
 M. 
 
 Macula acustica, 223 
 Malleus, 100, 231 
 Malpighian capsule, 345 
 Mammalia, 14 
 Mammary glands, 27 
 
486 
 
 INDEX 
 
 Mammary pouch, 28 
 
 Mandibular arch, 69 
 
 Manubrium sterni, 61 
 
 Manus, 126134 
 
 Marsupial bones, 121 
 
 Marsupial pouch, 28, -375 
 
 Maturation, 3 
 
 Meatus, external auditory, 224 
 
 Meckel's cartilage, 69 
 
 Mediastinum, 298 
 
 Medullary cord and groove, 149 
 
 Membrana tympani, 224 
 tympaniformis, 286 
 
 Membrane bones, 71 
 
 Membranous labyrinth, 221 
 
 Menisci of vertebrae, 46, 48, 49 
 
 Meroblastic segmentation, 5 
 
 Mesentery, 236 
 
 Mesoblast, 4, 6 
 
 Mesoblastic somites, 8, 66 
 
 Mesoderm, 4 
 
 Mesonephric duct, 341, 346 
 
 Mesonephros, 341 
 
 Mesopterygium, 122125 
 
 Metacarpus, 126134 
 
 Metamerism of head and body, 33. 65, 181 
 
 Metanephric duct, 346 
 
 Metanephros, 346 
 
 Metapterygium, 110 
 
 Metatarsus, 126134 
 
 Milk dentition, 245 
 
 Morula, 4 
 
 Mouth, 235, 239 
 
 Miillerian duct, 346 
 
 Muscular system, 135 145 : voluntary 
 and involuntary, 135 ; integumentary 
 musculature, 136 ; facial muscles, 136 ; 
 muscles of the trunk, 137 ; of the dia- 
 phragm, 141 ; of the appendages, 142 ; 
 of the eyes, 181, 216 ; visceral muscles 
 Fishes, 142 ; Amphibia, 143^; Amni- 
 ota, 144 ; muscles of the feather sacs, 
 21 ; arrectores pili, 26 ; ciliary, 210, 
 213, 214 ; of iris, 210, 214 ; cremaster, 
 375 ; lateral, 137 ; papillary, 316 ; 
 platysma myoides, 136; stapedius, 231 : 
 tensor tympani, 231 
 
 Myocommata, 137 
 
 Myotomes, 66, 137 
 
 N. 
 
 Nails, 26 
 
 Nares (see Nostrils), 
 
 Naso-lachrymal duct, 201 
 
 Naso-palatine duct of Cyclostomes, 74 
 
 Navicular, 133 
 
 Neck, 12 
 
 Neostoma, 155 
 
 Nephridia, 341, 346 
 
 Nephrostomes, 341, 345 
 
 Nerve-eminences, 190 
 
 Nerve-plexuses, 179 
 
 Nerve, lateral, 185, 187 
 phrenic, 141 
 
 Nerves, cerebral, 180 188 ; olfactory, 
 196 ; optic, 207 ; oculomotor, trochlear, 
 and abducent, 184, trigeminal, 184; 
 facial, 185; auditory, 186 ; glossopharyn- 
 geal, 186 ; vagus, 186 ; spinal accessory, 
 187 ; hypoglossal, 188 
 
 Nerves, spinal, 177, 179 
 
 Nervous system, 149 ; central, 149 177 ; 
 peripheral, 177 189; sympathetic, 189 
 
 Neural ridge, 177 
 
 Neural tube, 9 
 
 Neurenteric canal, 151 
 
 Neuroglia, 149 
 
 Neuropore of Amphioxus, 157 
 
 Nictitating-membrane, 217 
 
 Nose, external, 204 
 
 Nostrils, 73, 82, 197 
 
 Notochord, 5, 9, 3449 
 
 Nucleolus, 3 
 
 Nucleus, 3 
 
 Nucleus pulposus, 49 
 
 0. 
 
 Obturator foramen, 111, 117, 119 
 
 Odontoid bone, 46 
 
 (Esophageo-cutaneous duct, 275 
 
 (Esophagus, 238, 257, 262, 263 
 
 Olfactory organ, 196207 
 
 Olfactory scrolls, 203 
 
 Olfactory tract and bulb, 159 
 
 Omosternum, 58 
 
 Oosperm, 3 
 
 Opercular bones, 77, 79 
 
 Operculum, 75 
 
 Orbital ring, 79, 91 
 
 Organ of Corti, 230 
 
 Os penis and clitoridis, 384 
 
 Ossification, 71 
 
 Osteocranium, 69 
 
 Otic bones, 78 
 
 Otoliths, 222 
 
 Ovarian follicle, 347 
 
 Ovary, 347, 359375 
 
 Oviducal gland, 363 
 
 Oviduct, 346 
 
 Ovipositor, 360 
 
 Ovotestis, 367 
 
 Ovum, 2, 347 
 
 P. 
 
 Paeinian corpuscles, 195 
 
 Palate, 92, 94, 288 
 
 Palseostoma, 155 
 
 Palatoquadrate, 69, 75, 76, 78 
 
 Pancreas, 272 
 
 Panniculus adiposus, 26 
 
 Parachordal and prechordal cartilages, 67 
 
 Paraphysis, 155 
 
INDEX 
 
 487 
 
 Parietal foramen, 85, 91, 162, 166, 171 
 
 Parietal organ, 155, 171 
 
 Parorchis, 346, 350 
 
 Parovarium, 375 
 
 Pars acetabularis, 119, 120 
 
 Patella, 132 
 
 Pecten, 213 
 
 Pectoral arch, 106 
 
 Pelvic arch, 109 
 
 Pelvic plate, 109, 111 
 
 Penis, 377, 379-384 
 
 Pericardium, 300 
 
 Perilymph, 222 
 
 Perinseum, 375 
 
 Perissodactyle feet, 134 
 
 Peritoneal funnels, 235 
 
 Peritoneum, 235 
 
 Pes, 126134 
 
 Phalanges, 126134 
 
 Pharyngeal teeth of Teleosts, 81 
 
 Pharynx, 236, 273 
 
 Phosphorescent organs, 18 
 
 Physoclisti, 280 
 
 Physostomi, 280 
 
 Pia mater, 151 
 
 Pigment of skin, 18, 19, 20, 26 
 
 Pineal organ, 155, 159 
 
 Pinna of ear, 233 
 
 Pisiform bone, 128, 133 
 
 Pituitary body, 155 
 
 Pituitary space, 67 
 
 Placenta, allantoic, 9, 337340 
 
 Placenta, umbilical, 336338 
 
 Placoid organs, 30 
 
 Plastron, 34 
 
 Pleura, 297 
 
 Pleurocentra, 41, 44, 45 
 
 Pleurodont dentition, 243 
 
 Pleuronectidae, asymmetry of head, 81 
 
 Plica semilunaris, 217 
 
 Pneumatic bones, 93, 99, 130, 295 
 
 Poison fangs, 243 
 
 Polar cells, 3 
 
 Polymastism and Polythelism, 29 
 
 Polyphyodont. 241 
 
 Prehallux, 127, 128, 133 
 
 Prepollex, 128, 133 
 
 Prepubic process, 117 
 
 Prepuce, 382 
 
 Primitive steak, 6 
 
 Pro-amnion, 357 
 
 Pro-atlas, 46 
 
 Processus falciformis, 211 
 
 Processus vermiformis, 266 
 
 Procoracoid, 107 
 
 Proctodseum, 5, 235 
 
 Promontory of sacrum, 51 
 
 Pronephric duct, 311 
 
 Pronephros, 341 
 
 Pronucleus, male and female, 3 
 
 Propterygium, 122125 
 
 Prostate, 377 
 
 Protocercal tail, 41 
 
 Protovertebrse, 8, 66 
 
 Proventriculus, 262 
 Pseudobranch, 278 
 Pterygiophores, 103, 105, 122125 
 Ptervgopodium, 378 
 Pteryla, 21 
 Pubis, 114120 
 Pupil, 210 
 Pygostyle, 48 
 Pyloric ceeca, 259 
 
 Q. 
 
 Quadrate cartilage, 69 
 
 B. 
 
 Radiale, 126133 
 
 Radii of fins, 102, 105, 122125 
 
 Radius, 126133 
 
 Receptaculum chyli, 334 
 
 Rectum, 236 
 
 Reproduction of tail in Lizards, 47 
 
 Respiratory organs, 273 298 
 
 Rete testis, 346 
 
 Retia mirabilia, 280, 333 
 
 Retina, 208, 214 
 
 Ribs, 11, 5261 
 
 Ribs, abdominal, 56, 57, 58 
 
 Ribs, cranial, of Dipnoi, 82 f* 
 
 Rudimentary limbs, 109, 121, 127, 129, 
 
 134 
 Ruminant stomach, 265 
 
 S, 
 
 Sacculus, 221 
 
 Sauropsida, 14 
 
 Scala vestibuli, tympani, and media, 232 
 
 Scales, 18, 20, 26, 30 
 
 Scapula, 107109 
 
 Schizocrele, 8 
 
 Sclerotic, 210 
 
 plates, 213 
 Scrotal sacs, 375 
 Segmentation cavity, 4 
 
 nucleus, 3 
 
 of head, 66 
 
 of oosperm, 3, 5 
 Semicircular canals, 221 
 Sense-capsules, 68 
 
 Sense organs of integument, 190196 
 Sensory organs, 189 234 
 Septum, oblique, 292 
 Sesamoids, 136 
 Sheaths of notochord, 34 
 Skeletogenous layer of vertebral column, 
 
 36 
 
 Skin, 16 
 Skull, 64102 
 
 bones of, 71, 76 102 
 Somatopleure, 8 
 Spermatozoa, 3, 348 
 
488 
 
 INDEX 
 
 Spinal cord, 149, 152 
 Spines, neural and haemal, 37 
 Spiracle, 75, 277 
 Spiracular cartilage, 75 
 Spiral valve of intestine, 257 
 Splanchnopleure, 8 
 Spleen, 335 
 Spongioplasm, 3 
 
 Spots, blind and yellow, of retina, 214, 
 216 
 
 Stapedial plate, 84 
 Stapes, 100, 231 
 
 Sternum, 11, 56, 5861 
 
 Stomach, 236, 257, 262, 263 
 
 StomodtBum, 5, 235 
 
 Stratum corneum and Malpighii, 16 
 
 Sublingua, 255 
 
 Sub-notochordal rod, 235 
 
 Suctorial mouth, 73, 86 
 
 Sulci, 154 
 
 Suprarenal bodies, 370, 385 
 
 Suspensorium, 70, 85, 92 
 
 Swim-bladder (see Air Bladder) 
 
 Sympathetic, 188 
 
 Symphysis pubis and ischii, 114 121 
 
 Symplectic, 70, 76 
 
 Syrinx, 258 
 
 T. 
 
 Tactile cells and corpuscles, 195 
 
 Tapetum, 212 
 
 Tarsalia, 126133 
 
 Tarsometatarsus, 130 
 
 Tarsus, 126133 
 
 Taste, organs of, 193 
 
 Teats, 28 
 
 Teeth, 74, 78, 82, 84, 92, 94, 239250 
 
 horny, 73, 241, 243, 250 
 Testis, 347, 359377 
 Thecodont dentition, 243 
 Thoracic duct, 344 
 Thymus, 256 
 Thyroid, 255 
 Thyroid cartilage, 286 
 Tibia, 126131 
 Tibiale, 126133 
 Tibiotarsus, 130 
 Tissues, 2 
 Tongue, 252 
 
 muscles of, 144 
 Tonsils, 335 
 Tori, 226 
 
 Trabeculse cranii, 67 
 Trachea, 281, 283 
 Triconodont tooth, 246 
 Tritubercular tooth, 246 
 Trunk, 12, 
 
 Tubules of kidney, 341 
 Turbinals, 100, 200203 
 Tusks, 250 
 
 Tympanic membrane and cavity, 86, 224 
 Typhlosole, 257 
 
 U. 
 
 Ulna, 126133 
 Ulnare, 126133 
 Umbilical cord, 340 
 
 vesicle, 337 
 Uniserial fin, 122, 124 
 Unciform bone, 132 
 Uncinate processes, 56 
 Urachus, 340, 358 
 Ureter, 346 
 Urethra, 358 
 Urinary bladder, 259, 262, 354, 357, 358 
 
 of Fishes, 350 
 
 organs, 348 -359 
 Urinogenital organs, 340 385 
 Urostyle, 41, 44 
 Uterus, 363, 373 
 
 masculinus, 377 
 Utriculus, 221 
 
 Vagina, 373 
 
 Vas deferens, 346 
 
 Vasa efferentia, 346, 350 
 
 Vascular system, 299336 
 
 Veins, 299318, 322333 
 
 Velum, 275 
 
 Vent, 235 
 
 Ventricles of brain, 156 
 
 Vertebral column, 9, 34-52 
 
 theory of skull, 64 
 Vertebrarterial canal, 50 
 Vesicula seminalis, 354, 363, 377 
 Villi of intestine, 269 
 
 of placenta, 338 
 Viscera, 11 
 Visceral arches, 66, 69, 75 85, 88, 93, 
 
 94, 102 
 
 Visceral tube, 9 
 Vitelline membrane, 3 
 Vitello-intestinal duct, 9 
 Vitellus, 2 
 Vocal cords, 283 
 Vocal sacs of Anura, 283 
 
 Wolffian body, 341 
 duct, 346 
 
 Xiphoid process, 61 
 
 Yolk, 3, 4 
 Yolk-sac, 8 
 
 V. 
 
 Z. 
 
 Zygantra and Zygosphenes, 47 
 Zygapophyses, 40, 45, 47, 48, 49 
 Zygomatic arch, 100 
 
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